Thanks for your question.
I'm sorry to hear about what you're going through with your husband, and can certainly understand your concerns. It is unnerving that he would attempt to spy on your roommate, but at the same time if that was an isolated incident then it may not be anything to be overly concerned about. However, him putting his hands on your sister's butt is obviously extremely inappropriate and crosses the line in a much more serious way.
On the one hand it's a good thing that he is very upset with himself about this and understands that he took things too far. If he made excuses for himself like he did with the shower incident, that would be a lot more troubling. However, if these two incidents are the result of some personal problem that he doesn't feel like he has control over, it's important that he figures out exactly what is going on here. He may be completely dedicated to you, but if he doesn't have control over himself that is obviously going to continue to cause problems.
I wouldn't automatically assume that this is a sign that your relationship is struggling, but it does seem apparent that your husband needs to figure out what the problem is in order for your relationship to function properly. If he seems genuinely remorseful and committed to earning your trust back, that is definitely a positive sign that his mind is in the right place. If you or him are personally finding it hard to move past this point, that is where marriage counseling may be very helpful in getting to the bottom of this issue.
I definitely wish you the best with all of this, and if there's anything else I can do to help please let me know.