I need help with weight loss for my little sister?

my little sister is 11 5'1 and 116 pounds is this a bad weight??? My sister wants to be a least 100 or 110 is that healthy for her age and height??? Plus my mom doesn't let her go outside to work so some work out things she can do inside the house??? Plus my sister has a craving for friendly fudge swirls or fudge tracks and she wants to eat it every night she isn't going to give that up so if i give her small amounts of the ice cream is that better????