You must do all women to advance the story.
I'm not really sure about the sisters and half sisters but all the other women are needed.
Motel manager is needed for all the extra stuff for the motel and for help with the goons when they show up.
And so on, they all got something to do with the game and are needed.
So far the younger sister just gets you a new higher paying job and her boss I am not really sure if she is needed or not.
You need to be at least best friends with the older sister to get the booze from. ADD: She is also needed to be invited to the other family's dinner.
The younger half sister is needed to get a room in there house for MC to use as he see's fit.
Well all the girls in the 2nd family needs to be best friends for the room there.
So far I have only gotten to the kissing stage with the mom and the younger sister, the older sister tells me its not in the game yet. (((I may have the older and younger sister changed here because I have not really played there story very much. I have the redhead as younger and the blond as older but I think I have it wrong and the redhead is older and the blond is younger.)))