Dating to sister
To disappear the effects of such perpetrations use A part of what this is about for your parents is that each has yet to acknowledged his/her cause in the mess; specifically, that their daughters have had no choice other than to mirror the integrity of their parents. Most parents hypocritically espouse honesty but harbor between themselves. For you to present this ultimatum to him you must be willing to let him go.
Who taught you and your sister that it was OK to date married men and thwart others for selfish gratification? Both you and your sister have been unconsciously doing your best to make sure your parents suffer for life's communications (whatever they "did" to you). There is an in your relationship with your sister and both parents. People who are complete with each other, with whom there is an experience of love and respect, do not create these kinds of problems for each other. By choosing to experience the pain and hurt that will come from being willing to let him go you will experience a transformation.
For a second marriage to work he must be able to describe how he masterminded the divorce—so that you can tell when he's doing it again but with another part of what this could be about for your sister is that she arrogantly thinks that there were no consequences for deceitfully having sex behind the backs of your parents—and, for supporting Steve in having sex behind the backs of his parents and possibly behind the back of his then wife. He could not bring himself to tell you and your sister the truth, up front, so he did his "friend" act. We know by the results his leadership communication-skills produced. There is a way to effect harmony, however, it can't be done with your present leadership-communication skills.
Dating someone who supports deceit always always produces undesirable karma. It won't work for you to continue dating him except that he agrees (at your insistence) to complete 24-hours of coaching, therapy, or counseling, alone, without you.
Your sister would never have dated a married man knowing how much it would hurt and invalidate her-your parents, her whole family— if she had been experiencing love with any or all of you. 'Steve'." You were clever enough to relate with him in such a way as to be able to say, you "barely" knew him (it's called a covert—albeit possibly unconscious—seduction). Communicated responsibly it would read, "Knowing he was vulnerable and horny I seduced him." It says that your mind refuses to acknowledge the power of intention, the communications that let him know, with absolute certainty, that you were interested in him. —Thank you, Gabby We see now that when you asked your sister if you could "date" him, you two had so many withholds between you that you were not in-communication with each other and so you couldn't tell she was lying; more accurately, you didn't want to know she was lying. " "I don't want to risk upsetting you." "Can I have sex with him?
Interfering (divisively) with a couple in a struggling relationship is self-serving and unethical; "I'm in the middle of a divorce" is not an invitation to seduce someone who, most likely, won't or can't tell you the truth as to how they destroyed their relationship (most "victims" are stuck in blame).
That being said, she could have asked his ex, as you asked Another part of your sister's anger has to do with the fact that by dating him you invalidate her reasons for divorcing him.
It appears that you have bought into his blaming-victim story. Now is not the time for him to be in an intimate relationship.
My sense is that a part of what this is about for him is the fact that he has not acknowledged to himself, or verbally acknowledged to your sister and your parents, that he knows that how he entered into your family was unethical (cheating).