So my 6 month old baby Maggie got on top of my 1 year old Molly last night and started humping her

I looked at her vulva and it looks enlarged but I haven't seen any blood. Does this mean she is getting ready to go in heat? I also noticed this morning that Molly was sniffing Maggie's rear and she has never done that. I was going to get Maggie spayed after the 1st of the year but should I schedule her now in case or wait?