] How long has Mrs Tilney been dead and how much of their characteristics do her children owe to her influence, I wonder?

I wondered this same thing myself and came to the conclusion that their mother must have had a great deal of influence. Eleanor says she was thirteen when her mother died, plenty of time for their mother to have a calming and positive effect on her growing family. Also, she could have helped them to adjust and deal with the General's temperament. I can't imagine that Henry's sense of humor and natural wit could have flourished so well if the mother had died when they were much younger.