I'm so sorry to hear everything you're going through. I'm caring for my DF who's dying of cancer less than 2 months after my mother died of the same thing. I can't even imagine how much harder it must be to go through that process with a child. My heart breaks for you.
But I just wanted to say that, from everything you've said, you're doing remarkably well and I'm lost in admiration that you're managing to plough through, even with issues like this. Many people would've been crushed in your position so it's a testament to your strength that you're holding everything together so well.
It's such a difficult thing to lose a close family member and some of the hardest things to cope with are the things I never expected to be an issue.
Like you, I had so much interference from extended family members and friends, navigating people's "opinions" on whether they should stay at home, or eventually go to the hospice (Mum stayed at home, Dad chose the hospice - both times I've had people tell me I made the wrong choice but all I did was support their choices). Or what treatment they chose and how it was the wrong type, or how I should have pushed for more at the hospital... ad nauseum.
Not to hijack, I just wanted to say that I know how difficult family interference can be at such a raw and painful time. In my opinion, having to deal with other people who feel entitled to get aggressively involved in the process, was the hardest part because I just wanted to concentrate on my parent. I didn't want to be distracted with pandering to someone's feelings or pulled about by having to go visit and explain the same things over and over again to relatives who wouldn't bother visiting but demanded I go to theirs to deliver updates in person.
I came to the conclusion that this is just some people's way of dealing with it. They're not intimately involved so they exercise their hold when it comes to decisions they can cope with or understand.
It doesn't make dealing with it easier. What they're doing to you is unforgiveable when you need all the help and support to get through this and help the rest of your family do the same. Unfortunately, I don't think they'll realise that for a very long time.
Sorry, I just wanted to let you know that you are in no way unreasonable and you have to do whatever you need to for your own mental wellbeing throughout this. Including telling them to fuck off and mind their own business, if it buys you a little respite from the bombardment.
But, if it's at all possible, I'd try to let go of any anger at them because it'll just hurt you and they won't even realise.
I ended up categorising my family into two groups - the "reasonables" who I could talk to and get advice from or use as sounding boards, and the "snowflakes" who felt that they needed to be emotionally coddled by me because they were FEELING all the sads and I needed to respect that and help them.
Ultimately, I just avoided the snowflakes, ignored them or paid them minimal lipservice in the case of the borderline cases. Just try to shut them out as much as possible for the time being.
Can you ask your DP to get them to back off? He can tell them they're loved and they're welcome to visit but right now, you, your DP and your children need time to work through this together and cherish the moments you have left.
Your parents in law sound selfish and silly. I don't think they're intending to hurt you or cause you grief, I doubt they've even thought about it that much. Can you, in your mind, sort of write them off for now - just earmark them as silly and selfish and you wouldn't expect any more from them, so you can feel justified in ignoring their unreasonable requests but without having to feel any more anger (that'll hurt you like hell when you're this raw and exposed) just to maintain your own mental space and balance?