As this is your 3rd DC, your mum, like a lot of other people, probably doesn't realise that you feel all the excitement and concerns that you felt with your first.
She's probably going to your sister because it is her first.
It's a fact of life that as you have more DC's, you have less visitors, help, flowers, gifts etc. People become a litle blase about you having babies.
I often used to be amazed that my DM got so excited everytime a new grandchild was born.
She is still the same now she is having GGC, and GGGC.
My DD had her 5th almost two weeks ago and I was just as excited as when her first was born. I took time off work to mind the other 4 and we had a wonderful if tiring time.
Just ask your mum if she could possibly come to stay with you for the birth. If finances are a problem, perhaps you could offer to pay for her travelling expenses.
I also think that it is important for you and DH to be able to bond with the new baby knowing that the other DC are being looked after.
Hope everthing works out for you and congratulations on your new baby.