Mrspeelywally maybe I can answer that one on the OPs behalf as my mother also has a personality disorder and she also came to visit and help, sorry "help", in the early weeks after my twins were born.
I asked her to come because I was overwhelmed, because like the OP I had no other family (only child, dad died a few years back), and because, as my mother like most people with PDs has never seen the inside of a psychiatrist's office never mind been diagnosed, I continued to believe it was possible to have a normal mother-daughter relationship with her if I tried hard enough. She came to our house and behaved herself for about a day and a half but after that the strain of pretending to be a normal person was starting to tell, and since I was focused on the kids and not her, I wasn't able to do the sort of emotional smoothing over of every perceived slight she was receiving from, you know - the babies, the fucking postman, my mate who beeped her horn at us and waved out the car window one day - and she started to misbehave. It started with little snide comments about my weight, how I was looking, about how little I was doing compared to my DH, how difficult the kids were compared to me at the same age - and it descended into things getting broken and hidden, her going to bed at 5pm, that sort of thing. Because I'm a cheeky bitch, after about 6 days of this I told her to sling her hook and marched up to the train station with her, her bag, the bloody pram with my twin girls, and went and got the guy at the ticket office to rebook her train tickets home for that day. The OP was probably too polite to do that, and just managed through the 3 weeks, giving her mother the benefit of the doubt in that "at least she helped with the practical things".
OP, IAmNotAWitch has nailed it. It's about your son, or it will be ultimately. You have a duty of care to your son that you just don't have in the same way to your mother. He's going to end up in the crossfire eventually, you know that - he'll be one of the bad guys, or whatever the hell goes through their heads. Don't put him through it, don't put yourself through it, have a happy Christmas for once. People with PDs are like emotional vampires, they only ever seem to be happy, or even just comfortable, when everyone else around them is utterly miserable. It's not your responsibility to set yourself on fire to keep your mother warm.
(Oh and you'd be very welcome on the Stately Homes thread on Relationships as someone said above)