Ok, so here's my "old" answer combined with my new: Oh yes, there have been some good fights over the years when people get huffy (embarrassingly I think I'm the one who gets huffy the most ;). We are in close quarters at Bear Lake and the sleep thing has been rough for some...jet lag (especially with those coming from Europe) and new babies through the years...trying to keep kids quiet in the wee hours of the morning while everyone sleeps (I had quite a few years of that back in the day and so I know how it goes...). This year [which is really last year since I'm so late posting this] we had a throw up bug going around before the reunion started and we were trying to quarantine kids so it wouldn't spread through all 48 of us (luckily it stopped after going through I think eight kids...). Yes, there are some bumps in the road, but every year I do marvel that the stars aligned in such a way that every one of my siblings married people I genuinely adore. I don't know how I got so lucky to be related to them (and Dave's family too), but I don't ever want to take that for granted because I know it's not the norm. I always come away feeling like I better up my game because I don't want to be the weak link! :)
I can't speak for the in-laws, but I can relate what I think helped in our family to make us such good friends. Many of the ideas my parents infused into our family life are pretty awesome. I wrote some of them in a sibling relationship post, combined with some of the ideas Dave and I have infused into our family and I think I still wouldn't change those answers much. Things like having a "repenting bench" so kids can learn communication and repentance, humility and grace help so much (details about that HERE). Family testimony meeting was a huge one in my opinion. I talked all about that back HERE.
But really, the majority of things I think create strong sibling bonds are all wrapped up in that older post so I'm just going to link to that post...it is HERE.
The bottom line is that the younger-children relationships generally morph into the older ones.
BUT, that doesn't mean if your kids fight there is no hope! Never fear if your kids happen to be in a stage of at-each-other's-throats constantly, because quite honestly, some of the most "fighty" relationships in our family growing up are the best ones now!