I'd have to agree with DonnaL. It really also depends on how big the adult kids are. 2 women on one is not so bad but if they are on the taller side it will be crowded. My boys are 12/13 and I figure I have about 2 more years of making them share the pullout at best and that's a pullout in a one bedroom (when either someone else is in the sleeper chair or there isn't a sleeper chair). I've slept in the sofabed plenty, solo and it would fit 2 but for several nights could get really old. Then again if it was my adult sister and my parents were paying for it, it might be ok, I'd have more of an issue with my mom snoring than the storage or the sleeping! Only you know your family best.
Biggest issue though really will be storage and that's true of any of the studios with the exception of OKW which has 2 queens and is just bigger overall.