Please do not listen to the people who are telling you to shrink the mom's boobs. That is a terrible idea! I love that they are the size that they are! Huge tits like that are already in a minority in these games, and are hard to find. Plus there are already a ton of games where people are getting the smaller and more realistic tits. Why do we need more?
That said I have some other ideas for the other options, if you should decide to add them in later. Of course the poll could be the order in which the characters appear, rather than if they get into the game or not. So here are some of my thoughts and suggestions on what could be implemented into the game later on. If you so choose to expand that far.
First off, I think the sister should take after the mother. Should the sister be as busty as the mother? Not really. But she should still have a fairly decent set of big tits on her chest nevertheless. It kind of helps show the genetic relation between characters. So in my opinion they should be big, just not as big as Mary's.
Grandma is a great idea, if she is done right. Most people's grandma's are not something to jerk off too...but in a porn game, it could be pretty sweet. I like the concept that she should have big knockers as well, the same as Mary's. That way, you could later on have a wild, incestuous duel tit fuck. Imagine grandmother and mother both squeezing their huge tits together around the MC's cock, rubbing them up and down in tandem. Or maybe a triple threat if you wanted to throw the sister in their as well. That would be fucking amazing, to see all those big tits from family members surrounding that one cock.
The high School teacher would be a fun idea too, but I think you should change it to high school principal. To me that just makes it sweeter. Because it makes her more of a matriarchal authority figure. And I think, that could be more fun. It would be more like fucking the queen bee instead of just one of the worker bees. My recommendation for this scene is for the MC to be more dominate. It might get a little dark. But let's say this bitch of a principal, was a bully to the MC when he went to her high school. He can't stand this bitch, but long story short, he ends up in her office, and getting her on her knees while she gives him a blowjob while he sits in her chair. The same chair she use to sit in, trying to look imposing, as she would intimidate the students that entered her officer. Also he could bend her over the desk, and grab her by the hair, while shouting "Who's in charge now bitch?!" and she responds "You are sir!" A complete domination scene.
On the flip side of that, you could have a scene where one of the female characters are dominate. The crazy ex-girlfriend would be a fun idea. And I think she should be like this somewhat buff girl, who was an aggressive jock in high school and ended up taking a career in professional wrestling. She is taller than the MC, and bigger than the MC, and she ends up intimidating him into sex with her. Scared out of his mind, the MC complies to her demands, because he knows that if he doesn't, she will beat the shit out of him. So she ends up having her way with him, albeit it briefly. In my mind she would probably make him eat her out first, and then rides him cowgirl style while telling him to squeeze her breasts really hard. He does what she wants, hoping she cums soon just to get out of this nightmare.
I don't know if you mentioned this before, but Tina's mom should also be an option. I think she should be a super bubbly, overly energetic, aerobics instructor type. The kind of person who count's every single calorie, and won't shut up about diet and exercise. And the reason for this is that could be her job. She is a physical trainer, and after having some conversation with her, the MC gets invited to a private workout session because she really wants to make a bond with her daughters new boyfriend. To placate his girlfriends mother, he agrees and when they are alone in the gym. But when he gets their, he can't help but sport a massive boner, when he sees his girlfriends mother in a tight spandex outfit; that leaves little to the imagination. Next thing you know, she is trying to help him do a certain exercise, where they have to work together. But he inadvertently rubs up against her, and before you know it, we hear the porn music begin. And what started out as some weird exercise, ends up with him hot dogging her spandex clad ass, as well as whatever follows.
Finally I like the concept of dad's new fling! First off, I got to say, what jack ass would leave a woman like Mary? But I digress, it's still a good idea and I think you should follow up on the two traditional stereotypes for the fun of it. The first stereotype being that dad had a mid-life crisis, and got himself a sports car, and is trying to dress up and act, like he is twenty one years old again. Completely making an utter fool out of himself.
The second is the dad's fling. She is a complete airhead, who is half his age, with bleach blond hair, big boobs, small brains. She is addicted to her phone, make up, fashion, stupid reality t.v. shows, and finally social media. In essence, the typical bimbo stereotype of today.
A great scene for this girl would to have her bent over the hood of dad's new sports car, taking a selfie, while the MC pounds her up her ass.
That's all I got for now. I know it's another long post, but I hope it was helpful.