I'm not sure it's productive to try to figure out if feelings are justified. Feelings are feelings, neither good nor bad. And when you're hormonal and aware of the huge change coming to your life soon, they're going to be magnified. So go easy on yourself.
If I understand this, this is your FIL's 3rd grandchild. Your first, his 3rd. You are your husband's second wife, and sometimes there are inevitable comparisons that we second wives do to ourselves on top of what other people put on us. So sometimes we say, "How come I'm not good enough to deserve a visit?" That may not be fair to anyone, but it's something that happens a lot.
Your FIL has a new girlfriend, and this is their first Christmas together. Perhaps he has never met her family since they live on another continent, and perhaps Christmas is a big deal to their family. Sharing in their traditions may be really important. He also may feel that he wasn't much help and was perhaps an intrusion on the other births, getting in the way of the bonding with exhausted new parents. Maybe he thinks he is doing you a favor. Maybe hospitals and nursing moms freaked him out in the past. Maybe he thinks that you would like this time alone with your husband and newborn. Do you have other family who will be around? Perhaps your husband wants time to introduce his daughter to the new baby and make that a special time, and perhaps having Grandpa there will be a 5th wheel.
Since the flight is booked, I think you should take the long view here. You will have many years to enjoy family time. If your FIL winds up marrying or at least spending his life with this girlfriend, you're going to want her on your side. You will both be second wives and will have something in common. One of the most frustrating times is right after giving birth - you're tired, sore, overwhelmed, and trying to figure out the constant needs of a newborn. The revolving door of people who invade your hospital room and your house (often in upheaval) can be a huge blur that you don't entirely enjoy. Spacing out those visits is a good thing. I think, if you can go into this last month of pregnancy and the delivery with as positive an outlook as possible, and if you can cherish the first moments of your new family life without thinking of what's missing, it will be easier for you. And if your FIL comes home and feels he made a mistake and missed out, he'll make it up to you. Even if he doesn't regret his choice, he's going to be in your family forever - don't make this a line in the sand. As anyone with a Christmas birthday will tell you, this is going to be an ongoing issue of making your child's birthday a special time. You're going to have to make adjustments for the next 18 years. If you start out feeling resentful and shortchanged even before the baby arrives, however justified your feelings may be, the person who's going to have the toughest time is you. I'd avoid saying anything at all for now - let your husband handle his father, and don't get into this now with Christmas and your very special moments of happiness at stake.
I wish you a very merry Christmas, an easy delivery, and wonderful joys to come.