Did I miss somewhere how old this girl is? Is she capable of vocalizing? Because if so, I would definitely be asking her (in front of mom) to use her words to tell mommy things.
Even if she's little, I would also say (in front of mom) "Oh, ouch. Remember that we don't pinch. Pinching hurts!"
I can almost guarantee that if mom does think somehthing is up, this is her way of "asking" you. If it happens again, and mom has the same reaction, I would call her on it. "Gee, I haven't had any issues with her. She seems to be doing fine. Do you have any questions or concerns?" Doesn't have to be confrontational, but you may as well clear the air!