I feel like both of their mothers aren't dead. I think that someday they're both going to be reunited with their mothers, and the turtles will finally have a mother as well. On that one episode where they thought that creature was Aprils mother but it really wasn't, Leo after a while instead of calling Aprils mom Mrs. O'Neil called her mom instead. I think that Leo wants a mom. I think that all the turtles want a mom.