Coming from someone who has a similar video game addiction, I can tell you that video games probably serve a purpose for her that she isn't getting elsewhere. A feeling of accomplishment instantly. Most offline games or projects take longer to get that feeling of accomplishment and so it is easy for some people to get drawn into the video games. Non-gamers don't always understand this. That feeling boosts our self esteem and makes us feel good, even if nothing in the RL gets accomplished. The FB games are also simple, routine, and usually mundane copies of eachother. If you can find something offline that the three of you can do together that has those similar attributes, that might help.
As for the temper, that is something that she will need to work on herself and something tells me that the "mother talking" may not be the normal nagging, yes or no DH? Has she talked to someone about her anger and temper and things she can do to try to control it better? I'm guessing that she realizes that she has a short temper and that it has negatively effected her life. Does she want to do anything about it or does she rely on those around her to "baby her"?
The only thing I can suggest you and Miranda trying is to do your best not to use the terms and phrases that set her off. It sounds like she has a problem with someone telling her what to do in a way, although if she doesn't have a problem with "have to" I'm not so sure.
Life is about the journey and not the destination,
so what better way to know life
than to wander all the roads and paths set before you.