I'm so sorry about what happened with your mum, it must have been horrifying. I understand why you want to try and preserve your relationship with your father. It's devastating to lose one parent but the loss of two is hard to bear, especially in the time of life you are in, with a baby on the way - extended family become really important.
However, your father is mentally and emotionally abusive. He will have had a significant impact on your mother's emotional stability and his actions will have directly contributed to her breakdown and suicide. The girlfriend, whilst clearly a Grade A bitch, is secondary to all this.
He won't side with you on this. He won't give her up, one little bit. He didn't for your mother and he won't for his children. He is a deeply selfish and unfeeling man. You know this is true by the very fact he has invited her back into all your lives (not just his... which incidentally would be bad enough).
You have some hard decisions to make. There isn't a right or wrong answer but you need to ask yourself what is more important to you? Having your father in your life or not having this woman in yours and your child's life.
As others have said, you need to prepare yourself for the likelyhood of your father siding with the girlfriend if you try and assert any boundaries (which you are 100% entitled to do and I would not have that nasty piece of work within 1,000 miles of my baby if I were you).
What will you do if he refuses to keep her away? Would you be prepared to stand up to him and cut him off if he refuses to respect your boundaries? If not, how will you manage the emotional fallout of having her around you and the baby? Are you supported by DP and siblings? Could they be relied on to step in and advocate for your well being if you are not emotionally up to it (quite likely, post childbirth)?
You need to very firmly know where your line in the sand is, regarding contact with this woman and then have a plan of action that takes in all possible consequences.
Hard. I feel for you OP - good luck with the baby.