I agree - only acts matter. Your thoughts are yours to have.

OP, you write "I recently found pictures of my 15 year old step daughter along with other acts I found out she is doing. We talked and she said she was pressured to do them, but she may still be doing them. How do I trust her? "

She does not owe you reports on her sexual activity. If you advised her on the risks of pregnancy and STD's, you did your job. She might have said the thing about being pressured to do those acts in order to please you. Or she could have been afraid of you. Who knows? From your short message, without knowing the context of your relationship, it is impossible to say with confidence whether she was truthful.

But the most important thing is that you do not own her, that she is at the age at which young women used to get married in days long gone so her interest in sex is normal and you cannot control it. You can just advise/educate her in a reasonable, non-overbearing, manner, and hope for the best.