I'd be very forceful about it. If DP was okay with his mum and family waiting outside while I'd be in labour, I would have kicked him out.

She tried it with us, but I nipped that thought in the bud instantly. I even threatened DP with refusing to go to the hospital and I would birth at home and he would have to catch the baby. That fixed it.

It's downright abusive to be discounting your thoughts. ...they aren't the ones giving birth you are, and it is very rare for any mother who's just been through that to want anyone waiting outside...LET ALONE THEIR IN-LAWS!

It makes me really angry when people in the extended family assert some kind of ownership over the birth, if he or they don't listen to you, tell me and I'll come there fists flying

Seriously, assert your rights. you are being PERFECTLY REASONABLE.