I'd get something for my sister and for my BIL, either a joint gift or separate gifts, depending on what ideas you come up with.
Then, if you can think of anything that might be a good baby gift, you could do that as well. There are also plenty of "baby" gifts that are really for the parents to *use* for the baby, to make their lives easier. A baby carrier, for instance, or a swaddling blanket. Etc. I probably wouldn't get baby clothes, toys, etc. yet. I *might* get an ornament or something, but it wouldn't be a "baby's first Christmas" type of thing because it won't be, assuming the baby won't be born until after Christmas.