My guess is that we haven't met her yet. It isn't any past girlfriend. It isn't Robin. The roomate clue eliminates possibilities like Robin's sister.
I really think that if they tried to introuce the idea that it was robin's sister (partiularly if it was played by Cobir Smulsers0 the fans would go nuts.
It will break the train of the show and proves the writters have not reread their work. It cannot be Robin sister as she has not taken any of ted classes. Ted clearly stated in one of the epsoide that his wife was in one of ted,s class.
Actually how would we know if robin's sister (or anyone else) was in the class. But again I don't think they would try anything that contrived.
The mother could be Barney's half-sister, Carly. She and Ted have not met yet, and it is likely that they will meet for the first time at Barney's wedding. Not much is known about Carly, however it is known that she is in college, meaning it's possible she was in Ted's first University class. If Carly is the mother, then Barney would truly be the kids uncle, and Robin, being Barney's wife, would be their aunt.
Considering who the mother is, I doubt she cares if Ted spends time with Barney, and definately not jealous of Robin.I hope they tell us more. I was disappointed in what we saw. And just show us the goldarned mother already. I know some of you think the show has to end when we see her face but I would rather see who she is and how she fits into that crowd. Does Lily like her? Is she jealous of Robin? Does she wish ted spent less time with Barney and Marshall?