i think twins generally skip a generation so i wouldnt worry too much about that. Hopefully this happening will improve ur parents relationship with the woman in question.
Im not really sure what to comment with regards to the rest because I am very aware that families and friends rarely really understand what goes on in a relationship.
my ex used to have affairs and treat me quite badly..mostly with the aid of head games but he took great joy in reporting any of my outbursts and upset about any of this and somehow managed to omitt why i had behaved like this!! At one point i had a nervous breakdown and he took this as an opportunity to tell everyone how mad I am etc (cant argue against that too strongly tho

). he really made me sound like a gibbering nutcase and i had no idea he was doing any of this...until i fell pregnant. he left me for someone else when i was 3 months pregnant and told everyone the baby wasnt his bah blah...(mostly cos he didnt want his new girldfriend to know he had been with me when he was seeing her) and all of this only came to light because his family wanted contact with the baby. we had basically all been duped by him and made to believe different things.
im not in any way suggesting ur brother is anything like my ex. im just saying that its not always evident what is really happening in a relationship. u only hear one side and become protective of that person as they r already the closest one to you.
my husband now...in the start of our relationship, he did some things he shouldnt have (nothing serious, mostly just being a lying sod lol) but it broke trust and he started telling his sister how i had looked thru his emails and how much it had annoyed him. didnt say why i mistrusted him and now she treats me as though i need pity for being insecure. god it drives me barmy whilst he is all innocent, of course

lol.
on the other side of the coin...i HATED my sister in law for 10 years...as did my parents and sister (tho none of us knew how others felt cos we were all quiet and civil about it). she was rude, snide and back stabbing. it was nothing my brother told us that led to this...it was purely how she was when we were around. all said and done though...no matter how much she manipulated and lied to my brother...we all stood back and left him to it. he had every opportunity to leave her if he had really wanted to and we would have backed him up. just had to let him make his own mistakes. just like they all did with me and i made PLENTY of mistakes before any of my family had the pieces to pick up. sounds like this is how ur folks have been which is good.
Im sorry that this isnt good news for any of you but i really hope it helps the relationships get some kind of understanding and common ground. I know it did in my case. most of all...i hope ur brother is happy with the news. god knows me and my hubby have loathed eachother at times but i figure thats all relationships??
maybe this isnt helpful but just wanted to offer a different view that maybe if she is a witch, she has her reasons...not assumingly by ur brothers doing. or she could just be a right cow...in which case i hope pregnancy and motherhood will mellow her!!
xxxxxxxxxxx