Chloe, I envie you so much. I'd LOVE to have a mum that I felt I could lean on for help and support during labor. My mum is probably the last person I'd want in the room with me.. You're very blessed!
Two things come to mind concerning your DH-
One: if this is your first, it may be a looooong labor. if you had "back up" then DH could more easily do things like pop out to the loo, or grab a coffee and sandwich, or grap YOU a sandwich, or even a 20 min. snooze ect. ect. ect. I hope you don't have a 37 hour labor- but if you did... two people could "tag team" and that would mean less of a chance that your DH would be completely exhausted at the end when you really need him on his "A" game.
Two: They've done several studies showing that women who have other women helping them during labor are much less likely to have "interventions" i.e. emergency sections or assisted deliveries with forcepts ect. Why this is the case I'm not sure, but it's been documented in more than one case study.
But I don't think it is because guys "can't cope" I think it is just because men and women have different strengths and skill sets.
If you could have DH AND your Mum- I think you'd have a fantastic advantage of having the advantage of BOTH their strenghts and strong points, that hopefully would COMPLIMENT each other.
For example, maybe one is really good at helping you with breathing and the other is really good at communicating with the midwives/medical staff to ensure that you are getting what you want/need.