My sister's boyfriend is her future brother-in-law. Of course, he has to be nice to you. He often needs your help. But this is not a matter of course. You should thank him when you have the chance. This opportunity is very good.
My sister wants to give him a gift to strengthen their relationship. At this time, she will be a little unconfident and always feel that nothing is appropriate. In fact, you can suggest her to give a watch, a belt, a wallet, etc. If you think it is not creative.
I recommend DIY chocolate. Is there anything more suitable for couples than this? You can also make what you want to say. Couple cups, color-changing cups, and their photos are also very creative and practical. There are many other gifts. You can go and have a look. Help your sister choose a good gift. I hope I can help you.