You can easily train her to accept the car seat between now and December. Buy the car seat now, put it in your living room and do not mention it for a week.
After the first week put a new book and a toy on the car seat, offer to read her a book if she sits in the car seat, if she sits nicely you read the book to her, if she gets out of the car seat you put the book back on the seat and tell her you will finish it when she wants to sit in the seat. Eventually she will accept the seat as a place that she enjoys because she gets extra attention from you in it.
Maybe let her have one of her daily snacks in her car seat too.
She will be much safer in her own seat, and you will have a much happier flight if you do not have a squirmy 14 month old on your lap because you did not train her to like her seat.