You can prepare a 2 year old for a birth to some extent, for instance watching birthing videos (graphically appropriate of course). Throughout the pregnancy you can talk about the birth and practice making the noises you would in labour so that he isn't afraid, and so that he understands why you are making those sounds, and they are not sounds of pain. We did our best to prepare my DS1 who was 2 years/2 weeks at the time of birth and luckily my MIL was supportive of him being there (she was his carer as it was birth centre policy for the child to have a carer).

Throughout the labour he came in and out of the room, but i was in too much of a zone to really acknowledge him and he was more taken with the toys in the waiting room to want to stay much. But what was most important to me was having him come in as soon as his brother was born, literally seconds after, and that the baby see his face and vice versa. He was quite amazed by all of that. My baby made his way to the breast and fed, and later on i fed them both for the first time. I would have been upset had he not been a part of it, even in the small way that he was.

I hope your mother can come round to the idea... there is also a book about the waterbirth of a sibling, for children, i wish i could remember the name, its also great preparation for the young child. I will try to think what it was called.