“Stepmother” because the daughter is trying/plotting to get rid of her – like Bonjour Tristesse.
She has a near-incestuous relationship with the father and is with our playboy to antagonise him.
The father is an old friend and rival of the playboy – and they have a shared deep dark secret that could destroy them both in the eyes of those who love them. There’s plenty of conflict there.

(The father still believes the playboy is responsible for the death of a former girlfriend. However – the secret within the secret – is that it’s the father’s neglect and irresponsibility that led to her suicide. I think this is interesting because of the main tragedy/fireworks has already happened and only now are they dealing with this trauma that’s rippled back to them.)

Troubled – Um. I’m still working on whether he’s an alcoholic. He’s definetly a commitment-phobe. Like “La Dolce Vita”, he’s disillusioned by his hedonistic pursuits and searching for something deeper. He believes he’ll find it in the daughter – who he idealizes and puts on a pedastal. It’s only later does he realize her agenda (she’s using him and manipulating him to toy with her father). His arc: he’s a scoundrel who longs to be decent and honourable.

You say: must navigate a weekend retreat filled with sexual tension and jealousy.
Well there’s quite a bit happening.

You’ve got this playboy steadily falling in love with fiance’s stepmother.

This whole time he thought he was in love with this nubile young thing – who is actually using him to antagonize the father. More than anything, she wants to get rid of the stepmother who she resents.
The father is forced to entertain this old friend who he deeply dislikes – and is somewhat territorial and “jealous” of this former rival stealing his daughter.

The stepmother, trapped in a loveless marriage, steadily begins to open her closed door and be vulnerable.

But – it’s all the under the surface. I want to keep up the sexual tension and jealousy and mindgames until the third act where there’s this emotional bloodbath and everything is laid out on the table.

A real climax.

Anyways – cheers for reading. I’ll keep working on this.