Thanks again to everyone for your excellent advice, it's really helped me to see things from the outside in.
My mum's breakdown and all the surrounding hoo-haa happened almost 20 years ago, but after my dad left he moved in with someone who wouldn't even countenance me and my DSisters living with them. We were all studying and at school and at the time we didn't really have a choice but to brush all DM's activities under the carpet and try to rebuild a vestige of family life.
Of course it's all bubbled to the surface a few times since but DM has always been dismissive, saying it's an old chestnut that gets thrown in her face etc. I was very very internally angry about it for a long time, and can't say I had a good relationship with her until my DSes were born. She has been a good GM and loves my DSes and it has helped to paper over the cracks, but then something like this crops up and all my old rage, mistrust and lack of respect for her come back.
She clearly does have strange boundaries when it comes to sex. Thinking back, my DSisters were always allowed to have boyfriends to stay over when they were in their teens, including when they were underage. DM used to be overly involved in their relationships and bustups etc too. I remember her and DSis1 having flings with a couple of mates who were working in the town over the summer, both these blokes were in their 20s, DSis probably 15/16 and DM late 40s. These blokes were sleeping in caravans and tents and DM and DSis went with them.
My dad had had a gutful and was humiliated and had moved far away to get away from all the gossip and was embroiled with this woman. He just didn't want to be involved and I just felt too embarrassed to talk to anyone else.
I didn't really want DM or DSisters involved in my relationships so they never really met anyone before DH. I suppose being the OW suited me at that point because I didn't have any pressure from that bloke to meet my family! But I am so ashamed when I look back now. i can't believe that was me. He had a very nice DW who I knew, and a young DD and I just didn't care. I was just like a blow up doll for him and that was enough.
I remember being surprised by how kind and demonstrative DH was when we first got together. Just the way he kissed my head when he brought me a cup of tea, or that he cared when I was ill. In restrospect I was absolutely starved of normality!
When DH and I had DS1 and then got married, I think DM and DSisters were all really shocked. My youngest DSis was particularly weirded out by it, so much so that she immediately got engaged to the bloke she had been seeing for 3 months and also got pregnant. She shoehorned her wedding in 2 months before mine, in the same hotel, which at the time I was seriously hurt about. She split up with her H less than a year later and hasn't really enjoyed motherhood. DNephew has spent half the week at his paternal GM's since birth and she has admitted she has never really bonded with him.
I apologise for waffling but it's been really cathartic, so thanks to you all for reading. I had no idea when i started this thread that it would open up all this stuff. I didn't know how much it still affected me, or that it had rooted itself so much into my family.
As for the party, I am definitely determined that my DSes will not be seeing any burlesque one way or the other. Whether that's though confrontation or avoidance I'm not sure yet, but since the party is not until next year I have plenty of time to decide.
As for the masking tape question, I think it is something that is made for strippers. My DSisters call it pasties or patsies? One of them has a pierced nipple so it can't be that painful to take off. HTH!
Thank you all again for being so nice and so normal! 