Sir Thomas initially expects Mrs. Norris to take Fanny in place of a daughter, and he expects Mrs. Norris provide for Fanny and to leave her money to Fanny. It does not seem to me likely that he would provide money for Fanny to marry, if indeed he even could. His expectations are that Fanny either gets married modestly, for example a clergyman who could provide a comfortable home, or stays to live with them in Mansfield.

Also, neither Maria nor Julia had a coming-out ball, because they were to young when Sir Thomas left, so Fanny still might be to young to be expected to marry. Certainly this also would in general also depend on individual inclinations. I think there was not rush for Maria and Julia to marry, so perhaps they themselves would be just fine at home if it weren't for Mrs. Norris's officiousness.