This is tough, also the reason we chose a "family friendly" resort. My FI and I really wanted our nieces and nephews there (4 total). My sister said that she would totally understand if we chose an Adults Only, though. But her and I are very close and I talked to her about it extensively and ultimately left the decision up to her whether she wanted to bring the boys or not (before we chose the resort). However, we are now having issues because we don't want any more than the 4 kids at our wedding and ALL our family and friends have kids. So...now I have to tell people they can't bring their kids to our wedding at a family friendly resort, or I need to be prepared to have 25 kids to be there. I think that if you really want an Adults Only resort you should try to do it. I would just tell them what you were thinking and see what they say, they may surprise you. Especially if you are close. It just depends on how important it is for the niece and nephew to be there. Your brother may want a trip away from the kids. It's your wedding, but with family it can very tricky! Just talk to him. I hope it works out for you!