>>Technically Pixar, but I would present The Incredibles.<<
I am pretty sure that was pre-Disney but yes they are a cohesive family.
>>Aslo remember that movies are interesting because the characters face challenges outside of normal everyday life.<<
It is almost a running joke that all kids and characters in Disney animated features are orphans, adopted, etc.
More or less, yes. Tropes are tropes for good reason.
There are counter-examples, though, even in straight Disney stuff, for example Tangled which kind of subverts the trope in that the main character believes she only has a single parent when in fact her real parents are still searching for her. (Of course, "Flynn" comes from an orphanage, so the trope sneaks back in anyway...)
I wouldn't be surprised if a lot of it is due to the compressed nature of storytelling film in the first place. By limiting the number of familial relationships, the ones you choose to keep become that much more important and you have fewer distracting side-elements, which can be vital when you're talking only 80-90 minutes of run time for an animated film.