Early in the book it is mentioned that Mr. Craven's wife died ten years ago because she was reading in a tree and the branch she was sitting on broke. Later, it is said that Mr. Craven couldn't bear to look at baby Colin (who in the book's present is ten years old) because he reminded him of Lilias. The book never overtly connects the dots any more than that, and I must have read it a million times before I realized that Lilias not only died in childbirth brought on by the fall... but the fall itself was due to the fact that the branch couldn't support her weight in the later stages of pregnancy.
Martha also mentions when relating the story to Mary that Lilias was a small, petite woman, so it’s possible that she thought the extra weight wouldn’t make that great of a difference.
It's also entirely possible that the reason everyone's so concerned about Colin's health is because of the fear of damage from the fall and from the fact that his birth was premature.
Ben and a few of the other servants don't take much heed of Mary's tantrums at first. Martha is rather blunt about telling her off, something no servant would ever dare do in an upper class household. But as Colin is confined to his room and Lord Craven rarely stays long enough, the servants haven't had to bend over backwards to kiss their master's asses. Mary used intimidation to keep her servants in India in check but she instantly realises she won't be able to do the same with Martha - because she's grown up in a house where there's essentially no one to serve. Their orders come from Medlock and while she's stern, she's hardly as bad as Mary and Colin with their brattiness.
Martha's outspokenness makes sense when you realise she has never really experienced the chain of command in a household. She essentially takes orders from Medlock and there is no one else that outranks her or can reprimand her for speaking out of turn (as Medlock is too busy running the house and taking care of Colin).
And in the 1993 film Medlock keeps pushing for Mary to be sent away to a boarding school. Martha probably feels there's no point in indulging Mary too much if she's going to be sent away soon.
Mary still can't dress herself when she arrives at Misselthwaite. Perhaps she had a servant with her on the boat to dress her, but Medlock didn't bring one back to the manor because there wouldn't be any need. After all, Martha is a scullery maid who doubles as a chamber maid and lady's maid because there's so little work that needs to be done.
Mary's parents die slowly of a cholera epidemic, as do the servants. Mary was essentially alone in a house full of corpses for two weeks, where she had to scrounge for food (and she has never even learned to dress herself). The 1993 film is a little better, where it's an earthquake that kills her parents suddenly - and it's more likely there were other survivors who could help her immediately.
Mary's parents were so distant that in ten years they never mentioned that her aunt had died. Did she even know she had an aunt before her parents died?