Get a daysailer that four can fit reasonably. A boat that will fit Mom's annual family reunion will not be sailed, sitting alongside the dock until the reunion. And then it will be actually sailed by the few with most experience.
A smaller daysailer will allow all to try their hand with a more manageable size crew on board, ideally two. At this size, mistakes will be made and learned from, rather than just observed by bored relatives. It will also lighten the burden of all being willing to go sailing at the exact same time and willing to get along while learning to sail.
After a few years, all involved will have much more definite opinions on the next size boat up. For now, I'd stay under 27', preferably under 25'. That ought to about halve the cost of admission and, if this doesn't really work out, make it much easier to sell a boat that has become a source of contention amongst people you cannot ignore.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.