Help! I have a psycho stepmom!

Let me start off by saying that this behavior is recurring and often and this specific incident wasn't just a one time thing. This is what happened on 8/16/17 and serves as a good example of what I have to put up with. I am currently in college now and living in my own apartment. So here we go...

First of all she hid my package I had shipped to their house with my name on it from FedEx (not the first time she's done this either). I found it underneath some comforters next to the desktop computer sandwiched in between some cabinets. Then we were having a conversation about my tuition and she got offended about something I miss worded. I tried to explain and apologize but she said "screw you" multiple times, threw a hissy fit, stormed out the door, got in the car, and left. She came back and I tried to apologize but she literally shut the door in my face. Hasn't even said a word to me. Then she came over to where I was sitting and took the glass I was drinking out of. This lady is 65 and is acting like an irrationally teenager. I called my dad and he tried to rationalize the situation in her favor saying "well she's got a lot going on." Like, okay, but that's no way to act. I can't win. She's acted like this for 10 years. My dad always has something to say to back up her craziness and irrationality. I've always tried to be the bigger person but it doesn't get through. Another BIG problem is that he's left all assets, property, and bank accounts to her in his will. Everything. I'm not trying to act greedy or anything I'm just saying, like, that's a problem. I've tried to express all of this to my dad. He just says "you have to respect her. You have to show your appreciation to her. She's your elder. She's the lady of this household" and other things along those lines. I told my dad what happened and he said "just make it right and let it go" just like he always does. See what I mean?? When my dad got home she started to cry, threw another hissy fit, and ran upstairs. Keep in mind she's 65 and acting like this.

I honestly believe she has some sort of mental problem and I don't know how to tell my dad this without being offensive or pissing him off. I really don't know how to handle this situation anymore.