My niece

Ok my niece is a spoiled brat my big sister buys her everything there gonna buy her a cellphone and she only 8 years old I can’t believe it she growing fast she knows almost everything about shopping she know about roxy she know about coach I don’t even know that sh*t and when I went to there house she told me I was poor and I was like wtf so I told her so good I made her cry cause I don’t let her get away from anything me and my mom dad and brother don’t let her but than her parents get mad I don’t freakin care she is getting spoiled and so are her 2 brothers there also spoiled the 4 yr old boy when he doesn’t get what he wants he punches and pintches you and the 7 yr old just cries for everything like I know there great kids like when my niece sleeps over our house she’s a different person she nice but then the next day when my sister picks her up and takes her home and we visit them she is a spoiled brat and when me and my mom tell my sister and thing about her daughter she gets mad and I’m starting to worry about her cause my niece has a weight problem she is 14 pounds more she is getting obese and my sister isn’t helping her they buy her cheetos like every time they go to store and they go everyday they almost eat fast food every day and I need help I don’t want her to be obese and I don’t want her to get diabetes oh and when ever I try to talk to her she cries so I need help please