You don't need to fit in the seat if you're travelling with family, you can take up part of their seats too. You're only in trouble if you sit next to a stranger and they complain. Otherwise it might not be comfortable, but you will be fine.
Small steps. No, you won't lose a huge amount of weight by then, but if you start tomorrow, that's a big step forward. Accept you won't be a size whatever by then, see how much you can lose instead. It will be fine, and please don't let it ruin the holiday 