I could go both ways on this. if his immediate family is small (as it seems to be) and relatively close, then yes, I would invite her. It's only one more person. Especially if there won't be a shower for his side. If inviting her opens the door to "Well, then we need to invite ___", then no, don't invite her. or if you all aren't close to his brother AND there will be a shower for that side of the family, I don't feel that you need to invite her. (But if there won't be a shower for that side, even if you aren't close, I do feel you should still invite her).