I think you made a safe assumption, and I'm glad you checked with your sister, because "Suzy and Bob's Baby Shower" wouldn't necessarily have jumped out to me as a co-ed, bring the whole family cue. I think in the future, if a host is someone that close to you, it would be perfectly okay to ask, "Hey, is this co-ed or girls only?" Bridal and baby showers can go either way, I think, especially the more casual ones. Or you could ask about the general plan for the party--food, activities, etc. that might give you a clue. Just something to get a host talking about it. As long as you aren't saying, "Is Justin invited? He wants to come. Save a place for him next to me. And buy his favorite cookies." I think general inquiries are okay.