It is probably for the best that you feel uncomfortable around him, what happened even as a one off was not right. There is a boundary that he crossed from where there is no going back. Feeling uncomfortable and scared isn't nice but you need to feel like that to stay safe.

You tried to speak to your mom at the time, but she wouldn't talk, but have you brought this up with her since? I think you should try again, tell her you're scared for your safety, her safety, your sister's safety.

If your dad was having emotional problems that have since improved you still deserve to know why he did what he did, but I think that even if that is the case it doesn't excuse what he did and I think it is right that you don't feel safe around him.

Take care and make sure not to spend time alone with your dad and the same goes for your sister too.