QTake her reactions as a sign.

From Create Your Own Story

With her eyes closed and her breathing quickening your hand slides toward her knee and under the hem of her skirt. Your hand hesitates as if anticipating a scream and a lecture. Silence. The tips of your fingers lightly trace up the top of her left thigh and under her panties and rest for a moment on her hip bone. You feel her breathing become more intense, but still slow and her face begin to light up bright red and become decorated with glittering beads of sweat. You pause for a moment savoring the situation and then...

qRealize this is wrong and stop before you've gone too far.

qTake it a step further with your mother and cross that line.