Try the power

You think for a second, and call Penelope into your room. Because it is the morning, she hasn't changed out of her pajamas yet. Same as your mom, you realize. For Penelope, her pajamas are more of a white tank top, and pink panties. All you can see on your mom is just her robe. From what you can see, she might not even have anything on under her robe.

"I have a question for you mom."

"What is it Derek?"

"Will you remove your robe please?"

"Of course."

After she agrees, she removes her robe. Your suspicions were confirmed. She sleeps in the nude apparently. As soon as her robe was off, her surprisingly pink nipples grew hard.

"Mom? Why did you remove your robe?" Penelope asked

"Derek asked me to."

Thinking on your feet you come up with a solution to Penelope's question.

"Okay, from now on, whatever happens involving nudity, and sex, will be completely normal. If you see anyone else nude in this house, your inhibitions may be reduced, and you could probably get into sexual acts with one another. With this all, you will always be under my control at home. I am your master, as well as a part of this family. Do you understand?"

"Yes master," both girls say.