So what did you want to happen?
Stay, not go to uni, share the property with your sister X years down the line (which neither of you could have predicted)?
Her boot your sister out and you move back in, when you live elsewhere and have a mortgage with a husband? Sounds v practical I'm sure!
She could only give the tenancy to one of you.
Your sister could benefit from it. You, who have a property elsewhere and cannot run two properties, live in two places at once nor want to, could not (in any case as a non-resident for X years, chances are you could not have taken over the tenancy anyway).
You wanted your Mum to hand back the property and boot your sister out to fend for herself?
None of the alternative outcomes or possibilities would actually benefit you.
Your situation sucks and some of it clearly wasn't your choice or fault.
Someone got a bit luckier than you, in some respects (not all, did your sister go to Uni, get on the property ladder, get married etc etc...?)
That is life, give yourself a slap and move on eh, because this isn't doing you any favours.