If the other families find the boys overwhelming and/or obnoxious (loud, physical, etc.), then the brothers should say so to their sister and offer ways to get around it--places to gather where it's not an issue, for example. Big loud boys are much more fun to have around in big open spaces with a lot to do.
And, if your daughter finds it painful to watch the other two families gather without her, then she needs to say so to her brothers.
In other words, there's almost no role for you here except to urge your kids to talk to each other, as often as they bring up the subject to you.