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I'm sorry to hear about what you are going through!

As far as your period goes, know that it is a normal bodily function that you, of course, can't help! It's unfortunately a part of life and not everyone is mature about it. Your stepdad seems to be one of those people. Not all guys are like that, but I can understand it being frustrating that your stepdad is!

Your stepdad may watch those videos and talk to you about online predators because he is concerned about your safety and that's his way of expressing it. Really he should have a mature one-on-one with you to get your side of things as well.

I'd talk to your mom about the videos and the comments that your stepdad is making about your masturbating when you take too long in the bathroom. I'd pull her aside on a day when she is in a pretty good mood and ask her if you can talk to her without judgment. Then use "I statements" to express what you want to say. For example, "I feel embarrassed when my stepdad makes comments about masturbation/watches videos like that/shames your period." Explain the things that are being done and how they make you feel. Using "I statements" like I described makes people less defensive. I know it may feel awkward, but those comments aren't okay!

Another option you have is to write your mom a letter or email about what is going on. That way, you can get everything out without nerves or interruptions getting in the way. You can then leave the letter where your mom will find it or hand it to her directly. At least with this option you know that you'll have said everything you need to before anyone has the chance to respond.