I am trying to be nice, mostly so the kids see that I tried. They are coming to their own conclusions but I don't ever want them to feel like we didnt let them make their own choice, however I have explained to them that I will never let them be alone with her, or my Step Dad, ever... .
I guess I am just looking for some guidance, where would you go with it? Would you ignore it and stop any conversation regarding it? How would you answer back?
Other folk who have more experience than me with kids might be better placed to advise.
I can't help wondering what I'd do in such a challenging situation. Logically, my mind says that there would many things in the life of a 8-year old where they don't get the choice on a range of daily activities from bedtime to drinking a glass of wine, rather the parent decides for them and that's the end of the matter.
I guess I'm thinking that there may be the risk in these difficult situations of inadvertently putting the onus on the shoulders of the child without meaning to (and possibly even inappropriately seeking validation from the child regarding how one handles these delicate situations) even just at the subliminal level, and also thereby the risk of repeating the pattern of the child and adult role reversal. Not saying you are doing that, just there may be a risk.
My mother is also uBPD. I don't think there would be the remotest chance that she would set foot in a therapist's office, even if it was supposedly about me primarily. The self denial, the protection mechanisms, etc., would kick in and she would refuse to go. For me, attempting to link a "solution" to answering her question by setting that boundary condition would only stonewall.
Even if I could get her to enter the therapy tent either directly or indirectly, it would be highly unlikely that would produce a progressive outcome - more likely (in her case) it would lead to a retrograde outcome. But I'm talking specifically about my mom here and others
may have attempted this and had some success.
That said, your mom's question may still provide a positive opportunity. For example, if you have determined the set of boundary conditions you need in place to handle her generally and protect yourself and family number one ("only in my presence" and "only in a spirit of respect", etc) this could be a time when you could relate those boundary conditions to your mother: the moment any one of those boundaries are crossed the visit ends. She has been informed of your position and then you must act to enforce them.
Just a thought... .I now only meet my mother on neutral ground or at her house if at all... .either may I can simply walk out at any time if my boundaries are crossed. With that in mind it may be prudent to entertain the birthday / gift giving at a neutral venue like the local play park or zoo, etc: any overstepping and you simply pull straws and up and go. That has to be easier than having a scene over having to chuck out of your house if and when she has meltdown in front of the kids!
People with BPD aren't going to change. That your mother is capable of lunging at you during wedding party suggests an unfortunate reality. So you must be the one who holds the control, the sense of power. for the sake of the wellbeing of yourself and your family, whilst resisting any misplaced feelings of guilt, or shame, or social proprietary (that may apply to normal folk, but not those with BPD!).
If I had kids, I'd be actively building up their social support structure (turning to aunts and uncles, family friends in godparent role, etc) to provide substitute relationships as a back up in the event the matters with their actual grandparents soured to non contact. Maybe you are already doing this anyway!
Good luck, I do hope you find some winning way forward that works for you and your family!