Thanks for the ready answer and I like that you at least want to create sex scene that will be hot.
I can begin to express my opinion, other will continue.
Inspiring from famous tv series, like games of thrones you have a advantage respect them, you don't need to submit of big audience moralism and you have complete control over the actors.
Some scenes cannot be filmed or need to be showed with less particular possible, because it's more acceptable violence and torture even between friend and relatives than sex; and some scenes cannot be shoot because actor/actress don't want, the reason because we won't see another nude scene of daenerys.
From what I have read I think that you don't need to accelerate when the sexual encounter happen, you can create the condition and the setting before some characters begin to have sex, at the same time I don't think that you have to fear that too much sexual contents will ruin your game's story, if you prepare the situation and give personality to characters all will go great, even in chapter 1, where sexual part is nearly absent the way you have prepared the blowjob tell me you won't have problem.
If you are prepared to show, when the story will unravel, betrayal also between family and friend, death, etc.. you cannot worry about too much sex at that point.
I said this also because you said game with adult content (18+) so you create some anticipation, it can be good to wait when all the story is ready, but at some point one expect that the "fate" put the characters in some situations, and even if they think to do something wrong better remorse for having done that regret for not having done, otherwise, about the sex part, many will feel how having showed for sometime a beautiful cake without eating it, or maybe to have had the permission only for a bite without the possibility to have a bis, even if liked very much.
Speaking of relatives that are the characters that are more complicated to put in situation to fuck together and those you fear more to depict in cheap way, but at the same times are the first we see, have good images, and it's impossible in an adult game not thinking in sexual way:
Addine: It's for now the better model you have created, if the hero don't fuck her and more than one time I think that many will be unhappy, the good thing is that from what I have seen it's only a matter of time and the occur of the right occasion and neither of them would have great hesitation after.
Valeria: this is more delicate, it's the girlfriend of the best friend and the relationship seems more fraternal that with Addine, at the same time when the same Jonathan comment on her ass in the first encounter is hard not thinking of her in that sense.
Frankly if not for Drak's intrusion I don't think that this could happen, but instead the possibility at this point exist at least for a single fuck, but after having broken the taboo the things can be different, it wouldn't be the first time that someone can do something that think wrong because he likes the feeling very much, the taste of forbidden, and saying to themself that after all if none know none wil be hurt and there will be only the pleasentful part.
Aunt: this cannot think, it's too tied at the story we don't know, some hatefuck maybe in some situation.
Mom: If you don't want that someone asks you have to depict her like a monster, if hot it will happen, but in the case seen how the father treat her I can imagine a cup of wine too much, asking to the son if it has become a so ugly woman now and she is too old and after that something crazy can happen.
Naturally I'm not saying that he has to fuck Addine in chapter 3, 4, 5 , 6, ... or fuck Addine, aunt and Valeria in chapter 4, because this will be possible only if you put sex in front of the story, but if he has a sex scene with Addine in Chapter 4, one with Valeria in chapter 5 and another with Addine in chapter 8 I think that this can be more than possible.
This are example, but I want to say that if you put this moments in the story, they won't ruin it, but will add some instead, in addition to please some patrons, if you try to cut the sexual part for fear of be cheap it's better not doing an adult game, showing but not too much can obtain something neither flesh nor fowl.
Surely you don't have to exaggerate, prepare the right situation and events, show the character's feelings, but, aside that, the game will be better not worst for the sex part.
One thing that I appreciate on you is that you are really trying to create a good game and you aren't trying to answer in misleading way so to attract more money possible, even if at the end I won't support you I want to wish you all the better for the game.