Brother-in-law invites you to dinner for his birthday. Of course, you have to show some appreciation. After all, he is an elder and the one chosen by your sister. For the sake of your sister's happiness, you should also give her a gift, so that she will have face and not let your husband think that your family is stingy.
Haha, but if you are a student, it is another matter. But it is still recommended to give a small gift. If you don't know him very well, you can give him something that can be used at home, such as home decorations, which also shows that you are attentive. Haha, you can give them a pair of couple cups, personalized DIY color-changing cups, one for each person, which is very warm. It is also recommended to give them a box of exquisite chocolates, with "Happy Birthday, Brother-in-law" written on it, or write something funny like "Brother-in-law, don't bully my sister".
I still recommend the real-life couple dolls, which are cartoon statues made by hand with photos of my sister and brother-in-law. They are very embarrassing and funny. You can go online to order a pair. They will definitely like it. I hope it will be helpful to you, haha.