Female cat in heat acting like a male cat.?

I know I'm supposed to get the cats fixed, but their appointments are for after the new year, the vet is closed for anything other then boarding and emergencies until then.

I have inherited two cats from my aunt who passed away earlier this month. Both seem to be in this never ending heat cycle. They never had kittens, never mated with a male cat and both are females about 18 months old. Now one of the cats is acting exactly like a male. She is biting and even mounting the other female. I've checked the first cat twice and made sure that she is ended a female cat. (She just got out of a heat cycle last week.)

Anyone ever seen anything like this before?

And I swear, these cats are the first appointment on the first day the open after the New Year. I would have had them sooner, I just put it off with the funeral and everything that needed to be planned and now it's too late.