[A dog person answers!]
1)That the adult female doesn't stop the kitten is no surprise. Adult female dogs are equally tolerant of pups, concerning food, until they decide the youngster doesn't need such generosity and can look after itself, at which point they soon tell it off.
2)Don't leave food out all the time. What purpose does that serve? A future 'fatty' will eat every time it passes the bowl.
3) Feed them separately,so one can't get access to the other's food and as soon as each finishes take the remaining food away .Feed only the recommended amount of the recommended food each time.
The old one isn't going to starve herself (I assume she doesn't seem distressed or ill) but the youngster may well eat to much, too often, given the chance. She may be the feline equivalent of a Labrador, a breed notorious for overeating.
Sorry to read of your absentee. Cat people on AB may reassure you (as well as answer your other questions).