Looks like the contradiction I noted here isn't corrected, either.
So you don't have to click the link, here it is again:
Coincidentally, I was just coming to ask about familiars from the Reign of Winter Players Guide, too. My question isn't specifically about Winter Witches, though, just the familiar availability.
That guide isn't on the Additional Resources list at all.
However, the Additional Resources listing for the Familiar Folio says "the animal familiars on pages 24–27 and inside front cover, except the ioun wyrd and the petromin, are legal for play". Since the inside front cover of FF lists all of the RoWPG animals, they would seem to be allowed.
So the Additional Resources list seems to contradict itself in this case.