Oh god Lemon
That's a lot of baggage, some of it designed to make it so very much harder for you to have a frank "reality check" conversation than your average daughter/sister.
Does anybody else going know what they are going to see ?
Or is this going to be a big shock surprise to them ?
My instinct would be to attempt to pull DM ans SIS back from the edge in the hope they would start seeing their worth as so much more than the degree of sexual attraction they can garner. But your post makes it sound like the sort of task that could be beyond family intervention and more a case of requiring professional input before even a scratch could be made on the surface. Realistically you might have to wait for them to crash and burn before any of them are ready to reassess and move to a healthier position.
I feel for you love, I really do.
Your mum sounds like she may have similar issues to my own. She was dead keen on the idea of me glamour modelling when I was 16. Her much vaunted "pride" in my hypothetical "getting out of tits" pushed me close to a precipice in an industry that chews people up and spits them out. That was just one manifestation of her issues. And we no longer have a realtionship.
I am not suggesting you have to do anything as extreme as estrangment, but I can recommend the benefits of at least some emotional distance.
Because you cannot make people provide the heathy relationship you desire. The only thing you can control is the degree to which you keep hoping that desired heathy relationship will become a reality. And how you doggedly keep trying to make it happen despite all the evidence that they are in no position to give you what you need,
And once you get your head around what is within your control and what is not, it gets easier to step back from the aspects that are like salt in the wound, place the priority on the family you made rather than the one you come from, and render yourself less susecptible to manipulations, manuvering, emotional blackmail and "cheif whipping boy" status.
It's not a walk in the park, and it can be a very painful jouney to acheive the required step back. But it can also be very freeing and allow you to love people despite the things they do, while keeping yourself out of range of the sparks they create.
I don't think the cheif concern should be the boobies that the kids see. I'd be inclined to be more concerned about the one sidedness of the style of family realtionships you and your DM/DSis are modelling for them. I feel shit typing that, cos it doesn't half smack of kicking a woman when she's down ... and between a rock and hard place. But it isn't meant in a critical "wtf are you doing!" way. Nobody gives us a handbook on family of the imperfect kind before we get lumbered with them. It creeps up over time and establishes itself and makes it almost impossible to see due to a "deeply involved" lens.
It's a learned process to see things in more objective terms. You (general you, not specifically you) almost have to step out of your own head to be able to do it at times, so you can work out what you do and do not have any control over, and where you should be focusing the main energies to create change for the better.
I may be wide of the mark, it's all too easy to start projecting in this mode of communication, so feel free to ignore if it bears no realtion to the essence of what you are dealing with. But if any of that sounds like it might not be so far from your reality, it might be worth mulling over while setting aside the immediate distraction of the party boobies and granddad going purple in the face.
And ...
You've not been handed an easy family road to walk by any stretch of the immagination.