I am so sorry you are going through this. Whilst there is never a good time, the timing is particularly cruel.
I have been through this myself albeit I wasn't pregnant, about to give birth and didn't have any children thankfully. I know what you are going through, the emotions etc - to be pregnant and with a child at the same time, my heart goes out to you.
Whilst it doesn't feel like it at the moment, I can promise you he has done you a favour. Far better to know what he is like now and move on than to find out later down the line. You are strong and have a wonderful family, your brother sounds ace! Focus on looking after yourself and your babies and allow your family / friends to help.
As for the message that pathetic excuse for a man has sent - ignore it! Doesn't deserve a response!
When you see your midwife make sure your notes are updated to reflect that you do not want him anywhere near! I wouldn't even tell him when the baby is born!
All the best, you will look back on this in the future and realise what a turning point and lucky escape this was.