What to Write in a Christmas Card for a Brother-in-Law or Sister-in-Law

Writing a Christmas card for a brother-in-law or sister-in-law can feel surprisingly awkward. You want to sound warm without being over-familiar, and friendly without sounding stiff, and a plain 'Merry Christmas' can feel a little distant for someone who is, after all, family.

Below you will find ready-to-use messages for a brother-in-law and a sister-in-law, plus short options and a few written for a first Christmas as part of the family. Pick the one that sounds most like you and change a few words to suit.

Whether you have known them for decades or are spending your first Christmas together, you should find the right words here in a couple of minutes, along with a simple way to make any message feel personal.

Why the Right Words Matter in a Sibling-in-Law's Christmas Card

Brothers-in-law and sisters-in-law are an interesting part of any family. You may have married into the family, or your sibling may have, and either way you spend holidays together without always having much to say to each other in writing.

That is why a Christmas card message can feel a little awkward. A line that is too formal sounds cold, one that is too jokey might fall flat, and a plain 'Merry Christmas' can feel like you did not think about it at all.

The good news is that you do not need to write a lot. One or two sincere sentences are almost always enough, and the tone can be as warm, grateful or light-hearted as suits you both. Most people find that the shorter the message, the more natural it sounds.

Below are messages for a brother-in-law, then for a sister-in-law, followed by short options and a quick way to personalise whichever message you choose.

Christmas Messages for a Brother-in-Law

These messages are warm, friendly and relaxed. Use one exactly as written or adjust a few words to suit your brother-in-law, and add a joke if you share the same sense of humour.

• Merry Christmas to a brother-in-law who feels more like a brother. Thank you for always being there.
• Wishing you a Christmas full of good food, good company and a very long nap on the sofa. So glad you're part of the family.
• Thank you for the laughs, the lifts and the last-minute help this year. Merry Christmas, you star.
• Merry Christmas to a brother-in-law who always laughs at my jokes, even the bad ones. Here's to another great year.
• Family isn't just who you're born to, and I'm so glad you're one of mine. Merry Christmas.
• Merry Christmas! I promise to let you win at board games this year, but only once.
• Thank you for being such a good friend as well as family. Wishing you and yours a very merry Christmas.
• Hoping your Christmas is relaxed, joyful and full of everything you love. Thanks for being such a great brother-in-law.
• Merry Christmas to the one who always brings the best stories to the Christmas table. I can't wait to hear this year's.
• Wishing you a peaceful, happy Christmas and a wonderful new year. We're so lucky to have you.

Christmas Messages for a Sister-in-Law

These messages are sincere, affectionate and sometimes gently funny. Use one exactly as written or adjust a few words to suit your sister-in-law, and add a personal detail if you have a shared memory.

• Merry Christmas to a sister-in-law who feels like a real sister. Thank you for always being there for me.
• Wishing you a Christmas full of joy, laughter and as little washing up as possible. So glad you're part of the family.
• Thank you for the chats, the support and the very good advice this year. Merry Christmas, with love.
• Merry Christmas to the sister-in-law who always knows exactly what to say. I'm so lucky to have you.
• I'm so glad the family gave me a friend as well as a sister-in-law. Merry Christmas.
• Merry Christmas! Save me a seat next to you at Christmas dinner, because I have a lot of gossip.
• Thank you for making every family get-together so much more fun. Wishing you and yours a very merry Christmas.
• Hoping your Christmas is calm, cosy and full of everything you love. Thanks for being such a lovely sister-in-law.
• Merry Christmas to someone who makes every family gathering better. Here's to many more together.
• Wishing you a peaceful, happy Christmas and a wonderful new year. We're so lucky to have you.
• Thank you for welcoming me into the family with such warmth and kindness. Merry Christmas, with all my love.

Short Messages and a First Christmas as Part of the Family

When you are short on space, or simply prefer to keep things simple, a few well-chosen words work beautifully. This set includes a few for a first Christmas as part of the family, and each one works for either a brother-in-law or a sister-in-law.

• Merry Christmas, with love and thanks.
• Wishing you a very happy Christmas.
• Thank you for everything. Merry Christmas.
• Happy first Christmas as part of the family. We're so glad you're here.
• Merry Christmas to my favourite in-law.
• So happy to be spending Christmas with you. Merry Christmas.
• Warmest wishes for a peaceful and happy Christmas.
• Thank you for welcoming me so warmly. Merry Christmas.
• Merry Christmas from all of us, with lots of love.
• Happy Christmas to a very special part of our family.
• Merry Christmas. Family looks good on us.
• Wishing you a Christmas full of love and laughter.
• Merry Christmas. Here's to many more together.
• Happy Christmas. Thank you for being part of our story.

How to Personalise Your Sibling-in-Law Christmas Message

Any message above will work as it is, but one personal detail makes it feel unmistakably yours. Here is a quick way to add it before you seal the envelope.

1. Pick the message above that matches how close you are, whether that is warm and friendly, light-hearted or short and simple, and keep the wording close to how you would speak to them in person.
2. Think of one thing you are genuinely thankful for, such as a favourite family tradition, a kind gesture or a moment from the year that made you laugh.
3. Add one sentence about it, either before or after the message you chose, for example 'Thank you for helping us move house this summer, we couldn't have done it without you.'
4. Use the names or nicknames you normally use for them. It is a small change, but it makes the message sound like you.
5. Read it back as if you were the one receiving it. If it feels sincere and sounds like you, it is ready to write.

Frequently Asked Questions

What should I write in a Christmas card for my brother-in-law?

Keep it warm, friendly and relaxed. A line such as 'Thanks for being such a great part of the family, Merry Christmas' works well, and a light joke is welcome if you already share the same sense of humour.

What should I write in a Christmas card for my sister-in-law?

Aim for sincere and affectionate. Thank her for something specific, say you are glad she is part of the family, and add a personal touch such as a shared memory from the year.

Should I sign a sibling-in-law's Christmas card from the whole family?

Either works. A card from the whole household is perfectly normal, but adding a short personal line from you alone makes it feel more thoughtful, so choose whichever suits your relationship.

Whichever message you choose, a short and sincere line with one personal detail is all it takes to make your card feel thoughtful rather than awkward, whatever your relationship with your brother-in-law or sister-in-law is like.

If you are still looking for the right card to go with it, browse the personalised Christmas card collection below for designs for a brother-in-law or sister-in-law, from classic and festive to funny.

A kind word at Christmas is a small thing that means a great deal, and it is one your family will probably remember long after the decorations have gone back in the loft.