When a teenage girl realizes her big sister works as a prostitute to put herself through lawschool, she has to find a way to financially support her, without her sister finding out who the money comes from, or else she will not accept the help and continue with this self destructive work.

You cannot assume that everyone reading this logline will share your view that the sister being a sex worker is a terrible thing. I recommend you take the judgment out of the logline, especially the bit about sex working being self-destructive.

You can certainly have a theme that explores sex working being self-destructive but leave it out of the logline.

I'm sure the Pretty Woman logline would focus on Vivian finding true love rather than being rescued from sex working by Edward. Perhaps your logline could focus on the teenage sister finding a way to support her big sister through law school.