Hi Shirley,
It sounds like you are very upset and having a difficult time at the moment. It must be tough for you to be on your own with two young children, especially since you are pregnant.
It sounds like your husband is having a tough time too. It's very natural for him to want to be with his mother when she is dying. It's obviously a very tough choice for him, given that his wife is pregnant, but this is the last chance he will have to spend time with his mum. He may feel that he wants to care for her himself as a way of thanking and honouring her for all the care she gave him as he was growing up.
It's very common not to want to bring in outside help, even if it is offered. Perhaps he and his siblings could think about contacting Home Hospice (http://www.homehospice.com.au/). It's outside help in a way, but it's not professional nursing or palliative care or anything like that. They would send in a trained mentor to talk to your husband and his brother and sister about how they can best draw on their community networks to help them through this difficult time. They would still be looking after their mother themselves, but talking to the mentor may help.
Have you spoken to your husband about how you feel? When you say that you think your marriage is over, it sounds like you must have been upset for a while, maybe about other things? Do you feel afraid about something going wrong with the birth, or no-one being there to look after your other children while you are in hospital? Perhaps you have some friends that you could call on to help out - I'm sure they would understand, with your husband having to be away.
It's a really difficult situation and I don't really have any advice, except perhaps to say that it might not be the time to make any big decisions. This sounds like it is the biggest challenge that you and your husband have faced so far, and no doubt your husband really needs you now. It is always helpful to have friends and family with you when you are going through really tough times, so call on as many people as you need to and don't forget to look after yourself.
Sending you my best wishes.
Louisa