Should I just pay

I think both Gossipisgood and Goldmerchant gave good advice.

Any claim on a person's estate would be null once the estate is settled, so make sure you look after this sooner rather than later.

This. Tell SiL to sort it herself. Nothing to do with you.

He just needs to.go through his insurance.

It's not really your problem so I'd ask your SIL to maybe write your neighbours a letter letting them know the repairs need completing before xx date as the car is being traded in or whatever & could they possibly give a time when they can pay the outstanding money agreed to be paid. Say if they want to now go through insurance companies can they please give their details so you can start the process. Ask your SIL to make the letter as gentle as possible saying they appreciate it's a difficult time but unfortunately there's a deadline for the repairs to be sorted.

Hrft
You could pay for a quiet life but doubt anyone will thank you for it.
Be prepared.

This

I’ve never paid my excess. It’s always been covered.

SIL just needs to notify insurance and claim on his estate?

Flipping heck at 19 I was hit by a neighbours car and sorted it all myself never ever would I have thought to ask a family member to sort it for me that's just weird, let them stand on there own feet, of they want the money and fix then tell them to go sort it

I hit my neighbours sons car and we sorted it all out via insurance, at no point was his Mum involved

When someone dies any debts due are paid from the estate and if the estate is considerable an advert has to be placed in that London Gazette asking for anyone owed money to come forward.
I’m not suggesting this is going to happen at all, but, given that agreement was made to pay, the person’s son witnessed the accident and you are on good terms with them, I can’t see the harm in your mentioning it just once and making a decision dependent on the reaction you get.
If it’s an absolute no no then nothing’s lost and you can decide what to do next.

Just go thru insurance- leave it to them.

I'm not sure how your SIL's car insurance will react if he contacts them now, you're supposed to contact them straight away. It would be worth him finding that out first. Assuming that won't have a comeback on him its the most likely way for him to get the money back.

Even with the son of the deceased knowing about this I'm assuming the main reason your neighbour agreed to pay it was to avoid his car insurance premium going up, which isn't really an issue anymore. It's unlikely to be top priority for them to sort out even if they're willing to do so.

How long ago did the person pass? Have you any kind of relationship with the young person who now lives there?

I honestly think unless you're on the bones if your arse it could be too awkward. They could well just say 'I've no idea what your talking about', then you'd have to find proof the deceased agreed to pay etc.

Then of course there could be no money left in the estate Or the new neighbour could refuse to discuss the estate with you and or it might make them feel you're incredibly insensitive to come to them with this while they're bereaved.

Claimants don't usually have to pay the excess if they aren't at fault.

I’m baffled as to why you would be remotely involved in your adult SIL financial issues.

He should go through his insurance.

20-21 is not a kid. If you are old enough to be responsible for driving a car then you are old enough to sort stuff out when it goes wrong. And he is wrong for not reporting it to his insurance even if he doesn't intend to claim. In fact it could invalidate his insurance and make it very hard to get insurance in the future. You could offer to pay the excess if you wish, as if the other driver is found to be at fault then you recoup those costs so you aren't out of pocket. But don't get in the middle of the conversation - your SIL needs to be an adult here and sort out the situation.

Yes I think it's unreasonable to ask

If they really need to claim for it then I think they should go through the insurance

Op you need to stay out of it. Let you son in law go to neighbours himself.

SIL (usually means sister in law on mumsnet)

You tell your sil to go through her insurance. You do not pay it yourself

You go through insurance OP. Wash your hands of it.

put a letter through the door and write it from SIL
with all the accident and contact info and agreed monies amount
and of course condolences
perhaps in the letter you could advise that if paying the £ is likely to be a fuss then happy to go through insurance instead as easier.

You have 6 years from the date of the incident to claim for damage. Your sil would be best registering a claim- he can claim from his comprehensive policy but he will have to pay his excess and his insurers can pursue recovery from the neighbour’s policy. Alternatively he can claim directly from the neighbour's policy as a third party.

You say they are 20-21 year olds, but they are adult enough to get married and buy and insure cars, so they are adult enough to deal with this.

Your SIL has a written agreement to pay the cost. Of course, the person he had the agreement with is now dead, and the estate may not be required to honour it any agreements they made (I don't know the ins and outs of this). But it's not nothing.

In SIL's place, I would put a note through the door with my number and screenshots of the messages, giving my condolences and noting that you know this is a bad time, but asking them to get in touch because you'd like to know if they still plan to pay for the repair or if he should approach his insurers. Unfortunately for him, the other party's desire to settle outside of insurance is going to be reduced by the fact a dead person's premium can't go up. So I would be resigning myself to informing my insurer.

Because Op is a people pleaser or conflict averse.