Based on the plot content for the two episodes in question, I think it's obvious that The Witch is Missy and the Familiar/Apprentice is Clara. As @randal'thor points out in the comments, there are two likely interpretations for who The Magician is: The Doctor, or Missy (again)
The Doctor is The Magician
This is the obvious interpretation, and the one I assumed just by reading the title. The Doctor has been referred to as a magician/wizard before (e.g. "The Pandorica Opens"), and Clara would definitely qualify as his Apprentice. In addition,
Clara spends a large part of the first episode trying to think like The Doctor so he can find him, and The Doctor's actions show he has a strong relationship with Clara.
The only flaw I find with that line of reasoning is that
The Doctor doesn't treat Clara as an apprentice in that episode; they barely interact at all. If anything, he's far more concerned about her in "The Witch's Familiar".
Missy is The Magician
On the other hand, you could consider Missy playing both roles. For example, in "The Magician's Apprentice":
Missy actually guides Clara into finding the Doctor, and brings her along when they accompany The Doctor to his "death". Occasionally she tells Clara mildly-useful things but mostly Clara is just pulled along for the ride.
She acts more like a mentor to Clara in that episode than The Doctor does, and also doe something vaguely "magical" even for a Time Lord:
She returns from the dead without regenerating, and without explanation.
The main flaw here is that it doesn't seem to make sense to identify Missy as a Magician in one place and a Witch in another -- the symmetry between the two words seems intentional. On the other hand, Missy
acts very differently in the two episodes, both towards Clara and in general
so perhaps the change in terminology was intended to reflect that.
Witch and Familiar
I think it's pretty clear that Missy is The Witch and Clara is The Familiar.
Missy forces Clara into a Dalek shell and has Clara follow her around Skaro pretending to be a Dalek, then abandons her when Missy doesn't need her anymore.
Both of these things match up with the way a magic-user is traditionally seen treating their familiars (like tools.)