Well, played through the whole current game content, and there are several issues that I find jarring.
I understand MC and Deb had hard childhood, possibly even been molested by their mother (or father, or both), leaving MC emotionally stunted and socially awkward (bear in mind he's not some UK bloke Kinsgsman style, but upper middle class, so the bloke behavior and mindset should not apply) and Deb psychologically scarred, possessive and prone to predator behavior in order not to get hurt. Later in the story it becomes clear Milly's mom, Deb and MC are childhood friends, Deb is obsessing about MC, and actually grooms him to become hers (happening on her 18th birthday), however MC overcomes this pathological state somewhat and falls in love with Milly's mom. Unable to express emotion (and strangely behaving like beer chugging, pub hanging, working class drone from Croydon) he makes his family life a chore. At that moment, instead of setting him straight - predator Deb uses the emotional vulnerability and NTRs the MC - definitely not a loving but sick behavior (actually resembling a revenge on MC for choosing Milly's mom)
Aside from Deb not being only a emotional and sexual predator, her behavior would not pass in British armed forces, and MC's beta behavior would not stand in Her majesty special forces.
We have a conspiracy where a loyal soldier and comrade in arms would sacrifice his freedom for his family's safety and that actually holds water. The whole convoluted story of his wife hiding the daughter, making her hate the father, keeping in contact with his sister, marrying the gay senator... we have to wait the full story, because right now frankly is sounds like a bucketful of manure.
So we have MC, (again bear in mind emotional inadequacy does not equal social ineptitude, or rampant perversity, yet the dev has put an equality sign between them) that has 0 authority, the only growth we see is negative in his willpower, self-control, and emotional competency. The only positive movement is the 1 day (he breaks it later with Kimiko) he decided to be faithful to Astrid, and trying to assert authority (in the wrong way) over Deb.
Deb on the other hand, a visibly damaged person, is an open sexual predator, attention whore & jealous bitch. Her perverse affection (and I specifically do not think that about the incest part) with MC forces her to accost and seduce EVERY person she is afraid is competing for MC's attention (Astrid and Kimiko happened outside her view/reach/zone of influence), even the gay solicitor before he mentions he's gay. Not to mention she knowingly hid the truth about MC's family for SEVEN YEARS adding to his emotional downfall, and suffering of both Milly and MC. Her only redeeming act, and that would be because of her sick affection for Milly's mother, is to try to help MC address his emotional problems in order for Milly to accept her father. However those happen in the most inappropriate and damaging ways, possibly replaying the events from her own childhood, not to mention she knows she will not win MC's attention/love if the choice was between herself and Milly.
The choking scene with Deb is prime example of sophomore writer - it is definitely an unsubstantiated emotional reaction(MC never displayed even glimpses of such emotional outbursts), where the emotionless Spec-ops soldier would instead fall back to military behavior, called Deb to order and menacingly ORDERED her to stand down and check her behavior. And her being military trained, that would have instantly switched Deb off, dead in her tracks. Would have loved to see that scene!!!
What should have happened up to this moment in the game is the internal struggle and growth of the MC, opening step by step to people in the household, helping Milly conquer the fabricated feeling of abandonment by her father and betrayal by her mother, while mutually regaining/rebuilding emotional connections, struggling with mutual sexual attraction. MC shedding the domineering and controlling yoke of his sister, helping her along the way to outgrow the psychological scars from the childhood (with choice of staying in relationship with her or not).