Wow.
Question to those who think OP is unreasonable for being hurt, and have taken the "your responsibility, just get on with it" approach - is that how you would treat your own child, family member or close friend?
If a close relative of mine was expecting a baby, and needed help during labour / birth / pre and post-natal complications, I'd want to help. Having children is a fact of life. It's not some luxury holiday.
OP hasn't asked for help raising or funding her 4th child - she wants help just at the point of labour / birth, and for her husband to be able to attend with her. That is not unreasonable. Partners can provide incredible support during that time and keeping a labouring mother calm / happy massively impacts on positive outcomes for both mother and child.
OP's in-laws quite obviously favour their other children / grandchildren. Yes, it's their prerogative but it's understandably hurtful to OP. And it sounds as though this is the straw that broke the camel's back. There has clearly been zero consideration for OP and her husband, against MIL spending almost half a year, on the other side of the world, to support her daughter with childcare. The amount of people who say mothers often favour daughters over sons, as if that's justification, are completely twisted. My parents love me and my brother equally and I'd hate for them to favour me just because I'm female and he isn't. It's behaviour that is frequently demonstrated but it is not ok. Again, I ask those saying "it is what it is, just suck it up" - would you treat your own children in that way?
To OP, if the question was are you being unreasonable to expect the in-laws to travel home earlier, then I think there are a few things to consider... First, had the in-laws agreed to childmind for your SIL prior to learning you were expecting? If so, then they are in a slightly tricky position because they would have to renege on a promise to SIL and there are, of course, the logistical challenges of ensuring they are home 3-4 weeks prior to your due date so that they can quarantine for a week and be on hand if the baby comes early. Is this something that they have mentioned? I get the impression from your post that you would be open to reason but it's the apparent out of hand nature of the "no" that has irked you - as if there hasn't been such a consideration. In which case, I totally understand you being upset. The outcome could have been the same but if it felt reasoned with you, then the upset likely wouldn't be there. Am I in the right ball-park?
My brother and his wife are expecting a baby, very soon. My husband and I have a weekend away booked around that time (with my parents looking after our little one) - it's been in the diary since before the pregnancy announcement came. The timing means that the new arrival will definitely be here by then but, had it been the case there could have been an overlap, we would never have expected my parents to hold up their promise to babysit for the night at the expense of not being on hand for my brother, his wife and their new arrival. We'd have found an alternative (likely cancelled the trip because my parents are the only people who help us with childcare) because that's more important than our weekend plans!
I don't understand how people don't feel enough decency (let alone love) to want to help out their family when they're welcoming a new baby into the world. It's such a rare, intense, special, scary occasion and, I'd have thought, trumps pretty much anything else. Those who say you should just get on with it must be those sorts of people.
There's also an enormous amount of hypocrisy here - OP shouldn't expect help from others (she literally cannot care for her children whilst forcing a child out of her vagina or being cut open to have the baby removed from her body) yet it's absolutely rational for SIL to require help with childcare cover for the school holidays?
And to those who say the OP should have planned her due date better... Christ! You've clearly no idea how tricky conception can be for so many people.
I feel as though there are too many simple minds on MN who take the easy route of bashing an OP who's asked for help or is upset when there is favouritism, entirely on the grounds of "your bed, you lie in it", when I expect the reality is those people making such comments would actually help their family / friends because they're probably not the complete twats they come across as whilst OP-bashing.
Couldn't sleep, hence essay.
OP, take a deep breath. I can entirely understand why you're hurt but allowing yourself to stew on those feelings (no matter how justified) won't make you feel any better. Neither will considering future resentment (which is almost a certainty). You're expecting a baby and that is wonderful. Focus your energy on what will be a beautiful experience. Make plans with friends to help you and get your children excited for the adventure of staying with others. Take a step back from your in-laws for now, put a pin in the relationship and address your and your husband's feelings a few months after the birth (when you, your hormones and life have settled down). Ask your husband to speak to them openly and honestly - you feel as though they favour his sisters and it hurts. Whether they actually do, or not, or think they do, or not, those are your feelings and it's up to them if they want to try to alter those feelings. If they don't, then they should expect that you will behaving towards them in a way that reflects how they make you feel.
My husband had to have a similar conversation with his parents, relatively recently. It was basically on the same lines of the above, with the addition of "we've really tried and you've made no effort, we won't be making any further effort unless you step up". They didn't fully accept what he said (much centred around MIL who lies) but they did accept the sentiment and they've been making an enormous effort, since. They even looked after our (very calm, quiet and easy) child for a few hours (first time in 5 years).