Why does my mother in law do everything I do?

The very first day I met her we were talking about jean sizes and I was a size 9 at that time and she said she had some clothes that she didn't need and maybe I wanted them...I told her what size I was and I'll never forget that day "Oh I haven't been that fat in a long time" Gee thanks lady...We moved into our new house that we're buying...She's walking around looking in the house and says "And look what MY son has done" We get a new car...So she goes out and buys a new one the same day!!! We got a new sectional...So guess what she wants...A new sectional....I had a laptop when we first got together so she had to go buy one because I had one...I get on her facebook today and she's adding everything I have on mine!!! She's competing to see who has more friends...We used to talk for 45 minutes at a minimum a day...Oh no now it's 3 hours!! You know I have 3 kids and a life and I just don't understand her at times...I get lonely because my husband works second shift and she thinks I'm depressed and that I need to be on prozac like she did a few years ago...My kids have scarlet fever right now....Thought I'd call and let her know..."Oh if you would've called me I could have told you they have that cause my 2 sons had that" She always knows everything and she'll argue over it....Twinkle twinkle song...She argued one night with me and my husband and her husband that it's "Where you are" and we said no it's "What you are" She dug out an old book trying to prove us all wrong....Of course she wasn't right....What is seriously wrong with her????? My husband says now you know why I don't talk to her...You know I was gone with my mom out of town one day for a while and because I didn't answer the phone she called my mom's house looking for me...It's not the first time...Is she trying to make me crazy??? I have two kids that aren't my husbands and we have one together and when she talks about my girls she'll tell people "Oh those two aren't his, but they're our adoptive grand kids" That for one pisses me off and two hurts my feelings...We have been together for 3 years and married for 2 and she talks 90 to nothing and you can never get a word in with her...She knows everything and don't argue with her because you'll stay on the phone longer...So I just agree just to get her to shut up!!! She never got her hair done when I first met him, but now she goes and gets hers done because I do....Why does she copy me???? Any help would be appreciated.....