L.R.
It's been four years since your sister in law died. They are so depressed they don't answer the door (assuming you are certain they were home). Do you see huge red flags waving all over this situation?
This has long since gone far beyond your kids' questions.
Has your husband -- it's his job, he is the adult child here -- made any effort to see his parents in person and start getting them some help? You say that "that is a whole other situation" -- do you mean that he's making some kind of effort you're not describing here? I hope so. Because your kids' questions and hurt are very sad indeed, but not nearly as sad as his parents' mental states if they are holed up four years after their child's death. Tell the kids the truth: Their grandparents are suffering and need help. Calling and asking the in-laws "to make an effort" may be adding to the stress that keeps them away.
Please put the priority on being much more assertive with the in-laws about getting help. They may be dangerously depressed. One of them could be dealing with dementia or an illness and they may be hunkering down at home out of fear of anyone knowing. If I went to the home of someone I knew was there, and they did not answer the door for me, knowing I was their family and had just driven 40 miles to see them -- I hope I would have called the police and said I was concerned they might be trapped inside ill or injured. That could be the next thing that happens.
Please update the post and tell us if your husband is helping his parents get some mental health assistance. This sounds like a situation where maybe he needs to take time off, go up there for some substantial time, and arrange some services for them and possibly someone to check in on them daily. Maybe all that's being done already but it doesn't sound like it. Has your husband pursued a relationship with them since you moved there, beyond expecting them to pay attention to his kids? I hope so and you don't address that in the post. But if his focus has been on the idea that the kids will pull grandma and grandpa out of their funk, well, it isn't working, and he needs to move on to finding them some serious help.
If they say they won't take help or don't want to discuss things -- Sometimes the adult child has to get assertive with the parents for their own good, if they are ill in body or in mind.