How do I get her to see this behaviour?

alright so if you're going to comment "it's not your kid stay out if it" please read all the way through. We (my husband, myself, baby) live with my MIL. my MIL gets my niece (6, almost 7) regularly. well here is the problem. the neice is a HANDFUL to keep it nice. she lies CONSTANTLY. over little stuff! it is understandable for kids her age to lie but she is extreme. I think it's part of attention issues. because she will also hurt herself to get attention. or bring up sensitive topics to get attention attention. now the kiddo doesn't get paid attention to at home by her dad and step mom or her mom and step dad. so I get why she is doing this. but the problem is: I've tried talking to my MIL about the behaviour (she should not be told 15-20 times to do something). or the acting out. and to maybe do a bit of dicipining (such as time out or time away from a toy) to help counter some of this. or not give her the extreme attention when she hurts herself. but the MIL says she isn't changing anything because she "didn't get on to her boys over "every" little thing ad they turned out fine". well she did make her boys at least mind. and she doesn't want to stop giving attention because she doesn't get it at home. WELL she lies because she is getting attention even if it is negative and "getting I . trouble".and she is hurting herself because that's when she does get attention. but I'm trying to help my mother in law to understand that there is a healthier way of doing that.