HELP

hello all i have a problem.i can never get on with my partners sisters they are so controlling all the time and constantly taking advantage of my partner and treating her like a taxi service and when i tell my partner no she says im controlling but i wouldnt mind but first of all its time away from me and 2nd its my money going in for petrol.her sisters are really not nice people as they are very selfish spiteful and controlling.they always critisize my parenting over my son and im like little things like they will give my son lots of chocolate and then tell me his mouth is dirty and i cant look after him but when he is with them he is dirty and alowed to do what he wants and i wont let him round there house as last time he went he cut himself with a razor and he is 3 and beforw i picked him up and he had a huge kitchen knife in his hand so for safety i wont but you see i lose again as i am branded controlling for not allowing this.they hate me and i can handle that but they are constantly question me on where my money is going and teling me how to spend it and well it usually includes buying them something.i know this is my test in life but i have never come across such horrible people i have not even touched the surface with the things they have done to me.and well yes i have snapped on a few occasions but i try my best not too as i hate becoming this control freak but i dont mean it likke that.my son is the most important thing in my life and i would die a million horrible deaths just to protect him from hurt and pain.i have him every day of my life and dont really like him leaving me anyway so its even worse when i cant trust the people he is with.please if anyone has any advice please share.sorry for rambling.lots of love and light lightlover 247 xx