Sister freaked out on my nieces real mom

My niece is 6 she definitely is little bit over weight. She came cover saying she was skinny. My sister told she was. My niece whet on say her and mommy do yoga and only eat healthy to be skinny. My sister told her that’s good. My sister told her mom everything she said. Her mom say the boys in the neighborhood tell her she fat and says means things. My sister asked why is she hanging out with them still. Her mom said there friends she not talking she she can’t hang out with friends. My sister whet crazy. She started saying there not her friends, friends don’t make you feel bad, you need teach her everyone beautiful because if you don’t she gonna go school telling people there fat and now we have whole line of body problems, how you need teach her it’s better be alone then have friends that’s are just mean to you. My nieces mom got pissed, but I don’t think my sister wrong