Yeah, it could be possible that Gothel is actually Rapunzel’s mother. That would explain away why Rapunzel, who is most likely Victoria, doesn’t seem to actually be a member of the royal family. Gothel also seems to not get older even though both fake Hook, Victoria, and Alice are all significantly older then in this instance, so Gothel could still be young and be Rapunzel’s mother, like with Rumpel’s parents.
That would then make Alice and Rapunzel sisters. Victoria’s response to seeing Alice always seems to be extreme fear where she just ends up starring at her.