What to write to a sibling-in-law

A sibling-in-law card sits between friend and family, which is exactly why people stare at it for ten minutes. Warm and slightly funny is the safe register.

Forty messages below, from welcome-to-the-family to the tenth birthday card you have written them.

Brother-in-law birthday

• Happy birthday to the brother I did not have to grow up with. Result.
• Happy birthday - thank you for taking my sister off our hands.
• Have a great one. The pub is on me.
• Happy birthday, mate. Glad you are family.
• Another year of putting up with this lot. Well played.

Sister-in-law birthday

• Happy birthday to the sister-in-law who is really just a sister now.
• Happy birthday - thank you for improving my brother beyond recognition.
• Have a lovely day. You have earned the fuss.
• Happy birthday from all of us. Family gatherings are better with you in them.
• Wishing you a brilliant birthday and a lie-in.

Welcome to the family

• Welcome to the family. It is too late to change your mind now.
• So glad you are officially one of us.
• Welcome in - we are not as bad as we look at Christmas.
• Delighted to call you family. Genuinely.
• Welcome to the madhouse. You will fit right in.

Thank you to a sibling-in-law

• Thank you for looking after him so well.
• Thank you for the help with Mum this year. You did more than most.
• Thank you for stepping in when we needed it and never making a thing of it.
• Thank you for the hosting, the cooking and the patience.

Wedding day and anniversaries

• Welcome to the family - officially, and forever.
• So happy for you both. Look after each other.
• Happy anniversary to two people who clearly got it right.
• Congratulations. You have made him very happy, and us too.

Funny lines

• You married in voluntarily. That is on you.
• Happy birthday - officially my favourite in-law, no competition required. Shop this card
• Thanks for absorbing half the family drama. Much appreciated.
• Happy birthday. Still the best thing my brother has ever brought home. Shop this card

Browse all birthday cards

How to write it so it lands

• Sign from both of you if the relationship runs through your partner.
• Keep in-jokes to ones they are actually in on.

Where to shop next

• What to write to a stepmum
• What to write to a foster carer
• What to write to an uncle or aunt
• What to write to your adult child
• What to write to a co-parent
• What to write to your postie
• What to write to your cleaner
• What to write to a stepdad
• What to write to in-laws
• What to write to a neighbour
• What to write to a carer
• Thank you for a gift messages
• Just because messages
• Planning the party? Birthday party ideas
• How to make a balloon arch
• Photo booth prop ideas
• All guides

Frequently asked questions

What do you write in a brother-in-law's birthday card?

One light joke about how he joined the family and one genuine line. Keep it shorter than you would for a sibling.

What do you write in a welcome to the family card?

Say you are glad they are here and mention one thing you already like about them. Avoid anything that sounds like a warning about the family.