Save the complaints on dialog, script, unerotic setups, etcetera: This is plain-and-simple step-sisters - those naughty young girls. It has a built-in taboo element, although the studios more often than not do their worst to undermine that. But hope springs.
There is one way to botch this, which is to be porny with the girls acting whorish rather than sweet. Sweetheart probably gets that right, mostly, I would hope.
Mainly, I wanted to point and leer here at the young girl on the cover with the pink flower thing on her head. All I can say are the first two words in the human language, "Me want." What a doll.