Controlling Mom Gone Too Far?

Im A married 41yr old woman. I cant believe I still have this problem. I have 3 kids,16,14 and 12. needless to say we have typical teen problems in our house. My husband and I are at our wits end. My Mom and Dad offered to pick up our 16yr old son from school to "talk" to him. Couldnt hurt right? He came home with a box of candy to sell at school to make extra money..He also wouldnt share the nature of their"talk" to me.Told me it was his business.Next day I check his text( I know,,nosey mom,,but you have to these days) and I find a text she sent him that read..yada yada yada,,remember what i said,dont show this to your dad and erase this text after you read it.. i was sick!!! I went to her for help,,now she is teaching my son to hide things from his parents and be sneaky. She is always telling me and asking me,,when are you going to cut thier hair? ill take them to the dr appts. Buying them clothes because SHE wants them to wear that style,,She even told me after a recent haircut my son got "thats not the way I wanted him to cut it"..I finally got the nerve to call her and tell her she had crossed the line and I was taking control of my family. She reacted like I was killing her. Basically accused my husband putting me up to saying that and said I was disrespectful and not her daughter talking to her like that. I stood my ground for the first time in 41 years. I told her at the time I was not going to discuss it further. That was 3 days ago. Im sure she has had time to think about it. Now I just dont know what the next move is. I am a little weak and dont want to give into the controll of her again. any suggestions? I know its pretty sad and pathetic