LNM. Maybe a very straight unemotional letter to the sister and copied to brother along the lines of
We separated due to your admitted affair on (date)
I have decided to file for divorce and informed you of this (date)
You have approached me on x occasions:
List every single event that has occurred which you would not expect or tolerate from a stranger (because this is the status he now has)
I have warned you this contact is very distressing and threatening to me (this letter is being written for evidence not to make yourself look weak -I think you need to say this)
You have misled our DC
We are now moving into a phase where I will take legal steps to end the marriage. I would like to think that as responsible parents we can do this with our DC foremost in our minds to minimise harm to them.
If you continue to harass, force contact and x y z I will have no alternative to take steps to protect myself and DC from your behaviour. I have already consulted police and ?solicitors.
I really would like to think we can proceed with out undue unpleasantness because this situation of your making is distressing for everyone as it is.
(I've written this v quickly so its probably not great) but I would get formal and use this letter copied to family to get message across. I also think get going with divorce. English law gives you 6months to petition for divorce based on an affair or unreasonable behaviour. The theory is that if you have tolerated for that time you didn't find it unacceptable.
LNM. I rarely see friends. Between work and focusing on DC I don't get time. I think of them a lot but just don't physically get time