I'm sorry OP, this is stressful.
I'm assuming these are your parents you are talking about and you and your siblings are the executors.
There's so much to deal with here and I'd start by arranging an informal meeting with your siblings.
First of all I would focus on your parents' needs while they are still alive. Do they have lasting powers of attorney? Can they actually continue to live in their house or do they need to move into some sort of care home? (Avoid the kind of retirement flats with huge service charges like the fucking plague, they're impossible to sell. Realistically they either need to stay in their house with a carer if necessary, downsize to a smaller place with a carer if necessary, or move into a proper care home, depending on their level of independence and medical needs.)
Focus the discussion on these things first.
Then say, realistically, they're going to die at some point and when that happens we're all going to be bereaved and it will be hard to think about the practicalities, so what can we plan out in advance? Do we know where the wills are? How do they organise their finances and pay their bills? If they're still doing that for themselves and one of them dies, will the survivor be able to access money? Or do you need to do all that stuff for them now anyway? Do you have all the contacts you will need, for example, to notify the administrators of any pensions they have that they have died, and that sort of thing? Do you know what arrangements they would like for their funerals, burial, cremation, music, wake etc.
Then once you've talked about those aspects and worked out what you need to have in place, perhaps start to talk about wills and possessions.
From my own personal experience, when a couple leave everything to their children equally and don't make specific personal bequests, sorting out all their possessions can be an absolute nightmare. From a legal point of view, if their estate may be above the inheritance tax threshold, any objects of value should be valued and added to the estate for the purposes of probate. This is a very good legal reason for saying that when they pass away, people shouldn't just go in and grab stuff before it's all been sorted out. The proper way to do it is for the executors to go through together, get rid of rubbish, make an inventory of what is left, and store it somewhere safe while the estate is going through probate. In terms of the emotional side, you could suggest that in the immediate aftermath of their deaths, emotions are likely to be running high and people won't necessarily be in the best frame of mind to make good decisions.
So when the time comes, get the funeral out of the way, then the executors (and no one else) should go through and identify what gets kept and distributed to family and what you can get rid of. (You may want to say to the grandchildren that if there are any small things or trinkets they feel particularly sentimental about, to let you know so they don't get chucked, without promising that those things will end up going to any particular person. I know that the one thing I took from my grandmother's house would have ended up in the bin if I hadn't rescued it.) Then you do your inventory which should include anything of financial or sentimental value, including things like jewellery, family photos, any war medals or other family heirlooms, valuable furniture and so on. If anything needs to be valued, do that. Sort out probate. And then maybe six months later (not too soon after their deaths, but not too long either), have everyone sit round the table and look at what is there, and try to find a way of distributing it fairly, without falling out. It won't be an easy meeting but what is guaranteed to make people fall out is if certain family members just go and grab what they want before their parents/grandparents are even cold. Say you know of families who have been ripped apart by this sort of thing and you want to make sure it doesn't happen to yours.
It's horrible, but I firmly believe that if you take matters in hand, talk about it well before you need to, and have an agreed plan that you can put into action when the time comes instead of trying to make decisions when you're all grieving, you will save yourselves some heartache.