Once again...
Thing is, dramatically, in terms of the strctures RTD likes to use for these stories (including this one) his mother doesn't fit.
RTD likes to mirror characters and situations - e.g the Daleks have a crazed prophet and so do the Timeys. The Doctor's companions are mirrored with Davros' Daleks in Journey's End. You get the idea.
If this had been, say, the end of season 2, then the Doctor's mum would make sense - it'd fit with mirroring Rose getting a dad back. But in this, the Doctor's not being played against a parent - unless you count Naismith wanting his daughter to live forever, but then he's more bouncing off the Master's desire for immortality, and Wilf wanting Donna fixed.
Also, the Doctor's mum would not just undercut the show as a whole (the clue's in the name, right - Who) but immediately require a whole bunch of explanations. The only thing going for it is that the not-we might like the domestics.
Fannish-wise, FWIW, his mother would be human, and not a Timey. Plus it's implied that the rest of his family is dead as far back as the Troughton era, well before the Time War. Oh, and all the mums RTD has written in DW have been bloody annoying idioitic harridans, which this one wasn't - there's conclusive evidence for you!
So who's most likely? Well, she has to be a Time Lord who cares about the Doctor, so that rules out Rose, Donna, The White Guardian (not Timeys), the Rani (wouldn't care), etc... In descending order of likelihood, the remaining suspects are:
1) Susan - Lost on Earth, psychic enough to contact a human, married one, there's the signifcant look at Wilfs' granddaughter when asked who she is - i.e. "the Granddaughter".... There's enough evidence.
2) The Doctor's Wife (i.e. Susan's gran)- Could be a mirroring with Lucy's "faithful wife" thing as we were reminded in Part 1, except the Doctor's wife sets out to save him, not get rid of him. This would fit with the dramatic structure, and tease the fans (remember JNT's old jape? Then there's Patience/Larna) Also could fit with the look at Donna, as she's a bride at that moment.
3) Romana- Again, a Timey with a great fondness for the Doctor, and who's pretty much smarter than him. Often wore white too. Was President before Dalton
4) Somebody else. The Inquisitor perhaps- if only because she wore white, and might feel she owes him. But more likely it has to be family - a sister, perhaps, to fit in with all the other Star Wars knockoffs in the episode...