I have given gifts to unborn babies in the past, but I don't think it's necessary.
You either get your sister and BIL a couple gift or gift them individually. I agree that giving your sister a gift for the baby and BIL one for himself is definitely not the right option.
Since there will be no baby shower, I think getting something for the baby, or something that mom and dad can use for the baby separately, would be a nice gesture. But again, not required....especially if this "something extra" isn't in your budget.