Ok, my general advice would be, I think you should repair the GT as it is your sisters bike but it sounds like you wore it out. I think that is just good manners. Second, start saving up for a new bike, The GT is a mountain bike, but I suppose you want to get something a bit more for the street like a hybrid. There are a few good hybrids out there, Trek fx series, DS series and Specialized Crosstrail, and Cannondale all make hybrids, I have the Trek DS 8.6, but I had the 8.3 before, the 8.3 has disk brakes and front shock, it is a good value.

I would list out some things that are Must haves to help narrow things down, for example are front shocks a must have? are disc brakes a must have? I guess these depend on the type of riding you would be doing, asphalt, 2km off road and 8km on pavement, is that all? 40-50km, wow that is flying, I would think disc brakes would be a requirement, and probably the shock on the front. and some tires that will be good at that speed, aka not knobbies. Also is theft an issue? you may want to get something not too flashy. that is the one thing with my 8.6ds I am afraid to leave it out of my sight.

good luck with your search. I am sure some other forum members will have some good advice.