I certainly can relate to all the horror stories, and would feel the same way if someone complained about just about anything on a "free" trip. However, I am one of the few lucky ones whose first two
DVC trips, which were both with extended family, were great.
We bought DVC in 2008 with the partial intention of being able to take my DSis and her family (luckily Disney fanatics, too) along sometimes. Now, we have travelled together in the past, and know we are compatible. Our DHs are fabulous, very even tempered, and get along great. The four kids get along pretty well (5-12), but are 3 DSs and 1 poor DD, but she's a good sport. We decided at the beginning that we didn't have to do EVERYTHING together, and we'd let each other know our plans, and if they matched up great, if they didn't, fine, too.
Our first trip was FEB09 with a commando WDW trip at BCV followed by a 3-night Wonder cruise. We borrowed everything we could and bought extra points to make up the rest (next time I'd just pay for the
DCL - lesson learned), so DSis and family were only responsible for their park tickets and food at the parks. They were grateful and a lot of fun to have along.
FEB10 we rented a VGC GV, and invited DSis and family, and our one and only DCuz and his family (DW, adult DS, and her DPartner), so there were 12 of us. When we invited them, we sent a GV layout and explained they'd have one of the upstairs bedrooms/bath for the four of them. DCuz is not a DisFreak, but his DW goes every couple years with a friend. Again, we believed we would be compatible because DCuz and family have stayed at our home and are perfect houseguests - take care of themselves, pitch in, and totally go with the flow. Same deal on this trip - "here's what we plan to do - you're welcome to tag along or do your own thing." My DSis and I like to plan too much, though, so had to have ADRs for the 3 dinners we were going to share, and the DCuzs were grateful for that as they realized the benefits of having ressies for 12 of us.
Since the DCuzs only wanted to be there for 3 nights, we gave up the GV (the Paradise Pier View one, no less - it was AWESOME) and moved the remaining 8 of us to a 2BR for the rest of the week. Aside from my DS splitting his head open the first night at DL, and being woken up by jackhammering at 6 AM both mornings in the GV, it was a great trip. We hung out together some, and did our own things sometimes. And my DCuz (not the big Dis fan) sent a nice thank you and said he had soooo much fun.
Since then, my DSis has bought into VGC, too, so now all of our planning energy can be put into how to maximize our joint points! I know they're looking forward to being the "renters" so that they get the master sometimes; 2 BRs have worked well for the 8 of us in the past, but as the kids get older (and we're 3 DSs against 1 DD), it may work better for us to get 2 1BRs (I'm so spoiled now - don't know if I can do a studio!).
All of this worked, I believe because a) we had travelled or "lived" with all parties prior to the trips and knew we were compatible; and b) we had the understanding up front that there were very few expectations about doing things together, unless all parties wanted to; and c) because most, if not all of us, are "Disney Believers.
On my DH's side of the family, we'd probably never take them. He has 4 DSis's whom I adore, but none of them are into Disney, so I wouldn't force it. My DFIL and DMIL would think it was all very frivolous and not enjoy it at all either, so we don't even talk about it with them.

My parents are both passed, which is too bad because we did DL with them as kids and once as adults (before kids) and they would have enjoyed going with our kids. (Although my mom sounds like the LI mom who kept finding road blocks to going -
So if you even suspect there might be issues, I agree with most posters - don't do it! But also don't be totally scared off it if you think it might work - give it a go, and if it doesn't work, don't do it again. We're going on our DVC incentive cruise in November, with a few days at VBR, without my DSis and my kids are trying to understand how THAT can happen!! So it's what they're used to now.
Most of all - GOOD LUCK!!
