NC for this, mostly because I don't want anything I may have previously posted to impact on opinions on this issue. Also, get yourselves comfortable because it's kind of a long one.
DM has extreme form for erratic, spontaneous and ill-considered behaviour. I feel that in this day and age she would most likely get a bipolar diagnosis, but she won't engage properly with MH services or follow any recommendations from family/friends regarding this matter. She has left a trail of divorces and tempestuous relationships in her wake, and decided to walk out on Dsis and I in our teens to follow some Disney-esque dream of life on another continent. I fully believe that she intended for us to follow, and justified her choice to herself in these terms, but of course neither of us did and she has only been a transient presence in our lives for 15+ years now (this is perhaps a shining example of the behaviours she displays during a mania phase, some with long lasting ramifications).
She also displays some severe arrested-development tendencies in her emotional interactions with others, and will infantilise herself in a situation to try and delegate responsibility for her fears and anxieties. I remember her discussing all of her relationship issues with me as a child (I was referred to as her 'little psychiatrist') and it is only as an adult in a healthy relationship that I have come to realise how seriously fucked that this dynamic is/was. Needless to say, I have been left with a mess of social and general anxiety issues which I am just beginning on the road to working through, with professional help.
The point of all this is that, after all this time, DM is now talking about moving back and integrating herself into family life more. Recent interactions with her have confirmed my suspicions that she has made no improvement on her boundary issues, including (trigger warning) discussing her suicidal impulses in front of 2-year old DD and telling her that 'Grandma is just having a bad day'. I wasn't there to hear this personally: DW reported back to me, I would have come down on it like a ton of bricks if I'd heard it firsthand. DM has literally no self-awareness of the damage that this foisting off of your emotional baggage onto children who should not be exposed to, or feel responsible for, such matters can cause. Whilst she presents the outward image of a guilty martyr, she is in a lot of ways spectacularly selfish.
My instinct is to go into protective mode for DC: I absolutely will not have them exposed to the same shit, unhealthy dynamic that I experienced as a child. Frankly, I do not trust DM to know where the limits lie if she had them alone for a day/afternoon etc., as she has suggested she might when she is living nearby again. She definitely loves them and the rest of the family - I'm not querying that - but I am also a walking testament to the fact that that is really not enough. Whilst I obviously cannot and would not stop her from returning home, WIBU to prevent her from seeing the DC?