ok, well I think the problem with reverse AIBU as you never really put it from their point of view, you put it from your own as you wish they saw it.
Look at it from their point of view more - your SFIL has a holiday home he can't sell, right now it's sat empty for the bulk of the year, yet they prefer being there than in the UK. It might well have always been their dream to one day live there, have they talked about that in the past?
His father has recently died after a long illness and he's realised that he's probably only got a short time of good health left - if they are going to go to live in the other country, it could well be they've realised it's now or never. While your next child is his step son's child, that doesn't mean he's going to be as caring as if it was his own. Did your MIL get together with SFIL when your DH was young?
Meanwhile, your MIL is facing looking after her elderly MIL who has the money for a home but doesn't want to go in one. Her BIL doesn't do anything because his SIL doesn't work, yet she doesn't work due to her own ill health - even if they don't do much for GMIL, doesn't mean they aren't feeling the pressure to step up and do more and are aware if they stay it will fall to them.
Possibly she is also aware that if she wants to live overseas, she's probably got an even smaller window of opportunity to do it than her DH before she's too ill. Plus her DH is grieving and it's very easy to agree to do something like this to "fix" his hurt without thinking that she'll miss her DGC growing up. Even if your SGF's death was expected and more of a relief, it doesn't follow that his son isn't upset.
In at way at least, they are doing it in a most sensible way, by renting out, not selling their UK home, they can easily reverse this if it turns out to be a hideous mistake. They can trial it for 6 months then change their minds. This way, they'll be going out just at the worst time, winter when the other holiday home owners aren't around, there's not the same English community, they will see the realities of living overseas, not just being on a long holiday quickly going at this time of year.
For you, as her DIL, you need to think about what you are and are not prepared to do, I'd laying on the table now with your DH and his family, you are not in a position to take over looking after his Step-grandmother. If she needs someone to look after more than her carers are doing, then she needs to go in a home. Be blunt - it's your DH's step father's family, you can't be expected to do it. If no one is, call social services, but don't just step in and take over, because once you do, it'll become your problem and your DH's step-Uncle will never step up, and your MIL/SFIL won't feel there's anything they are needed to do.
you know they aren't people you could rely on anyway, they don't seem to really want to be involved in your DC's lives (not all grandparents want to be, not every likes spending time with small children/babies - it's not a popular position so often the 'not interested' people feel they have to fake an interest). So it's not like you are going to be missing anything.
Make it clear that you will not be able to have them stay for the first year after DC3 arrives, be quite blunt about this too - they have the money from the rental of their UK home, they can afford a B&B.
It might be worth for you seeing the positives in this situation - they've removed themselves from the various family situations in which you'd have to factor them in while secretly knowing they wouldn't be a help. IMO them being gone is just going to be easier than getting frusterated at people not being helpful when you think they should be and they could be, now they can't be you won't find their lack of help/support as hurtful.