Does Winston remember that his mother was a party member?

I don't think his mother was a party member. Winston's dreams of his mother are fragments of memories from before the revolution. He tries to recall the painful memories because at least he can feel something. He recalls a vision of his mother and sister drowning falling beneath the water.

Yeah, Winston's mother never was a party member, Winston's memories of his mother are indeed fragments from before the revolution, he so very much wished to have as many links with the past as might be possible!