I'm from a family of 5, with 3 very active brothers (all are adults, and all are very active in a variety of sports and physical activities, still even as very busy full time parents & workers).
Your kids are old enough to understand that there are different rules at different places, and with different people. So, I would just be upfront with them about your expectations when you go on vacation, as to their behavior. And what "setting an example" for the 15mo cousin--that he's little, they have to watch out for him, that he'll try to do what they are doing, and can't in some cases or might get hurt, etc. And set limits (no X in the house, YZ outside or in the pool are okay, etc.).
Then, tell your family before you go that a) you're all really looking forward to seeing everyone on vacation, b) you know that some family members have concerns about the boys behavior, but that they are normal, active boys who have high energy and that as long as they are not doing anything dangerous/hurting anyone, that you would appreciate their tolerance of the boys needs to exercise and burn off energy in high energy play, and that c) you had a talk with your boys, about how to play appropriately with their 15mo cousin and when playing with him, they will try to keep their energy more contained/play safefly; however d) as a 15mo, he is going to copy and its up to the other adult family members to make sure that the little baby is playing age appropriate activities, not the 5 & 7yos' job.
I have to say, I'm one of those people who don't really like loud rambunctious kids (see, just by my choice of the word rambunctious, I show my bias.... sigh). But I do try to keep my mouth closed when it comes to my family's kids (I have one nephew, man.... he played a game for years as a little boy, called "Knock Down Daddy" where my brother would kneel and my nephew would run at him and try to knock him over.... yikes.) I said plenty to my DH about my nephew (I do think his parents didn't do a good job guiding him as a youngster, but they weren't the worst, I guess). But I never said anything to him unless he was endangering someone else (like his younger cousins), for example, by running amok through the house swinging a stick -- and in that case, I sent him outside to play. I also would've have been less likely to judge him as much, if I'd seen him helping out, putting away his toys, acting in consideration of others--which he hardly ever did/does.... Seeing your kids do considerate things for the other family members would probably go a long, long way, to mitigating their negative feelings, imo! :)