I’m Not so sure DH is “lovely”.
he has a sister problem. His sister is sending deliberately provocative messages to him. What is he doing to tell his sister, that whilst he loves her, he does not want to hear those messages, read those messages and it is down to him and his sister to get to a point where they resolve whatever it is between them that means she has no boundaries in saying stuff about his loved ones that is nasty, unpleasant etc
he needs to call her out. Not the OP, not his parents (nowt to do with them , they’re just trying to prevent the NC shit)
Until DP does this , he is giving permission for his sister to keep on doing it. Going NC is not going to do anything accept punish him, his parents etc. a marriage will struggle if one partner has to effectively NC their own family, including members that haven’t done anything wrong, to keep his partner “happy”.
NC on MN is seen as a right, a badge of honour, a “I’m protecting myself” . In practice, unless there is actual abuse (vs sending stupid childish spiteful emails) , then it helps no one. It makes things worse. It causes huge distress and has impacts on other members of the family. It is actually hugely controlling behaviour and is partly done to lash out. Fair enough the OP not being with SIL, BUT that should,d be temporary, until her DP has worked through this with his sister and made her realise that he has boundaries, and she is making herself less lovable, less likeable, and looking pretty foolish to continue this campaign. He needs to tell her he will not listen anymore - which he is and that is where the problem is. Him going Nc with his family to appease the OP is simply avoiding his responsisiblity to sort this out properly with his sister.
nope , not lovely. Emotional immature. Irresponsible