Control part1

I was left seething today because of an incident involving my mother and my sister. My mother often butts in and meddles with my parenting style in front of my kids which I find highly inappropriate. She accuses me of treating my 2 kids differently alot. She's right about that I do. They are two different people so I respect their individuality and treat them accordingly. I don't love one more than the other I just love them differently.
I took my kids to visit her today because she complains if I don't bring them over. I can count on one hand how many times she has come to my place to see them but that's a different blog for a different day. When we walked in she had cooked and my son said he was hungry. He wasn't really hungry but kids always say stuff like that. She fixes him a plate. Then she pulls out something to give my other son. I asked her not to give it to him because I was fixing spaghetti at my house for dinner. My mother ignored me and gave it to him anyway. At this point my son admits he didn't want to eat his plate so I made him eat it. I stood over him and fed it to him. Derek has aspergers and some days he doesn't want to eat. I don't let him starve and there have been many days I have had to stand and make him eat. No big deal to us. My mother didn't like it and first she says "you didn't make Donovan eat it" This is her attempt to say I love Donovan more than Derek. I told her I didn't have to make Donovan eat because she gave Donovan something I said he couldn't have and under minded my authority. At this point she had nothing to say and comes back with "I don't wanna watch u force feed him so take him home"
A discussion in which I tell her how I parent my kids is none of her business. It really isn't any of her business. I could see if my kids were in danger or neglected but they aren't. Aside of typical little boys stuff I have really good kids. If my parenting wasn't up to parr they wouldn't be so good. Everywhere I go people tell me how good my kids are. My mother essentially said since she cant butt in and tell me how to raise my kids then to take them home and not bring them back. I think thats unfair to my boys. I dont like their daddy much and I never keep them from him. I know she was mad and frustrated when she said it but it should had never been said. The whole situation is completely inappropriate.
My sister doesn't like the discussion because my mom is dead wrong and cant win this one. She decides to butt in. We had a huge fight in 2009 where she picked a fight with me and she didn't win. We have not spoken since even though I did buy her a gift that year. So until today we ain't spoke at all. She then tells me that I abuse my kids and she was gonna call CPS to have them taken away. That's when I lost it. She is 25, lives at home and cannot take care of herself yet she wanna pass judgement on me. She works everyday, sees my mother struggling and doesn't even offer to pay a bill. I'm beyond mad and threaten to whoop her ass like I did 2 years ago at which point she picks up her phone and threatens to call the police.
Those who know me know I'm a pretty laid back and even tempered chick. Her saying she wanted to get my kids taken away from me she went entirely too far.
When is it acceptable for someone who doesn't have kids to pass judgement on the way one raises their kids? When is it acceptable for anyone to butt in?