Question to poll: is it okay for the mom to do this?

So my boyfriends mom always takes away his things (phone, game system, etc) when ever his younger sister gets in trouble. For example, his younger sister breaks a few rules and does some things she’s not supposed to, so his mom will ground her AND him because she says “it’s only fair”. But when he gets in trouble, his mom doesn’t ground his sister with him. Does anyone see this lady’s point of view? Does any one agree with her? I’m trying to see her side of the story but I don’t get it at all..

Younger sister is 12...