I don't really have advice for you but I can say that I've been where you are now. It wasn't a relative, though -- it was my neighbor.
"Gladys" and I have children that are about the same age, and we both became stay-at-home moms at about the same time. It was nice having a friend for me and friends for the kids so close, and we got together a lot.
There were things about her personality (particularly as it related to her parenting style) that grated on me but I never said anything.
As time went on and our kids grew older, the little things that bugged me grew into huge obstacles to our friendship. She still views her children as a burden that have been dumped on her, and looks for any opportunity to dump them on someone else -- her husband, me, any neighbor that happens to be outside. She says things (yells, really) to her kids and in front of her kids that she shouldn't, and doesn't think twice about saying those things in front of my kids. She now constantly belittles her husband in public. She belittles her son, but thinks her daughter is a little perfect princess. She so obviously favors the daughter, who at the age of 5 has perfected all the sneaky, mean, clique-ish techniques we all remember from middle school.
I finally had to make a decision about that family. My husband and I decided to cut way back on our exposure to them. We didn't see a way to drop them completely but we don't get together like we used to. The change in my kids has been amazing. I hadn't realized how much of Gladys' toxicity had been rubbing off on us!
I guess what I'm trying to tell you is that if you think there's a big difference in values now, that gap way widen as time goes on. I know in my situation, I always figured Gladys would learn new ways of parenting or keep an open mind to different things, but that didn't happen.
Hope this helps.