Yes. I think there's this big fear these days that saying anything to a young/teen girl about her body or weight will lead to a lifetime of "disordered eating," because that's the narrative everyone seems to be pushing. But you have to talk to your sister about this, especially since it will literally never be easier for her to lose the weight than it will be now. And talk to your mom, too; don't worry about offending her, this is your sister's life we're talking about here. She's got to get the fatty junk out of the house. You can't expect a kid who isn't even a teen to make great food choices all the time, so not having the option is key. Is there any way your sister can spend some time at your place over the summer? Is there a community park with a pool or gym or something near you? Maybe you can get her a membership and she can go swimming every day, or there will be fitness classes there or something? Our local park has all kinds of fitness programs, some of which are specific summer programs for kids. Might be something to look into?