This post gives you ideas for warm messages and funny quotes that you can write on your brother-in-law’s birthday greeting card. By definition a brother-in-law is your sister’s husband, wife’s brother or husband’s brother. But if you treat him like a real brother, the word in-law will never cross your mind. Whether you are a brother-in-law to him or his sister-in-law, celebrate his birthday like he is a part of your first family so that you create lifelong ties and a bond that lasts generations.