bloody MILs!!! They're a bunch of trouble makers 8-) . I think its perfectly natural to prefer your own mother over someone else's mother. Ask your MIL how she would feel if her daughters didn't prefer her over thier MILs. She would probably have a heart attack at the very idea. Or if she liked her mother better than her MIL.
I would never have had my MIL at my birth but I might have asked my Mum (except that when she was my sister's support person she was so interested in the birthing process and all the machines/equipment that she kept forgetting to support my sister).