Helen acts as the housekeeper and maid and cook, grateful for a place to live, so she's sort of an authority figure for MC, but not his stepmom/mom, not related in any way, not even by marriage.
Patreon do not care about the official title, while be seen as incest any family-like setting ; what include a foster family by example.
And here, as you present it, Helen is a motherly figure for the MC. The fact that she's not involved with the father will not make a difference ; she's the person that act like a full time mother with the MC. Add to this the fact that she live in the same house than him, and there's high risk that they qualify this as being incest.
It would probably have more chance to works if she wasn't living with them.
She's still the best friend of MC's mother, but instead of letting her come live with them, the father helped her to move in the house/flat next door. It's at her house that the MC goes after school. She was his confident when he have his firsts crush. Twice a week the two families are having a dinner together (once in one house/flat, once in the other). They past part of their holidays together, and things like that. But in the end, since the two families aren't effectively united, she's as a motherly figure as could be the mother of his best friend.
During my youth, I shared most of my holidays with the daughter of a friend of my father. And I can assure you that even with her being a real slut (sleeping with two/three guys each holidays once sexually active), the idea to sleep with her never crossed my mind ; she was like a sister to me, and I was like a brother to her. Therefore, this setting would works, being psychologically taboo, but technically not.
With a little more change in the setting, you can even make this have started before the mother's death. What would help to increase your chances with Patreon, since Helen was a motherly figure even when MC's mother was still living.
Helen and the mother feel like two sisters since college, and when the opportunity came, one decided to become the direct neighbor of the other. They still had dinners together, just less often. Time to time, when MC's parent had to go somewhere, he was staying with Helen for the two/three days of this trip ; same for Helen's daughter when Helen and her husband had to left the town. And they already past part of their holidays together. At this time Helen was an aunt-like.
This relation started to become deeper after MC's mother death, with Helen slowly switching from aunt-like to mother-like. And it became even more deep few times before Helen's divorce ; taking care of MC was a way to distract her mind from the shit storm that her husband started to put in her life.
Finally, after the divorce, Helen started to become closer to MC's father. But, due to their long time shared relation as two friendly families, they haven't made this official, not being sure how the children would react. Therefore, when the game start, their intent is to wait for their children to live their own adult life before they start to effectively live together as a couple.
Be noted that this second approach present an advantage. You can use Helen/father relation to put the incest into the game.
After all, if Helen was aunt-like, and now is mother-like for MC, this also imply that MC's father was uncle-like, and now father-like for Helen's daughter. What mean that their relation have some kind of brother/sister incest flavor.
Perhaps that putting some scene where they talk together about this can be a good diversion. Technically it's not incest, they are adult, in there discussion they explicitly say that they aren't related at all ; "would my late wife haven't been your roommate in college, that we would surely never met". But in the same time, they were really close families ; "think about all this time our families spent together. Do you remember MC's fourth birthday, your daughter cheering him to blow out the candles ? MC probably see you as some kind of aunt... perhaps even a little more since my wife's death..."
But this incestuous relationship is effectively placed outside of the relation that the MC will have with Helen and her daughter. Psychologically for the player is will be the same, but technically you'll never explicitly say that MC will bang is mother-like and sister-like, it's her mother's best friend and the daughter of her mother's best friend.
