DIL won't listen to anyone

If this sort of funeral-related gathering is normal, this may be another reason why certain sections of society seem to be more likely to fall ill with C19.

Having lived for many years in Muslim countries and worked with many Muslims there, I have to say that there was a not uncommon attitude or belief that it was all down to the will of Allah. Regardless of what we might or might not do, if Allah decreed that this or that would happen, it would - and vice versa.
I can imagine a similar attitude in the circumstances the OP describes.

Has anybody suggested your dil may need grief counseling?

pinkym I have sent you a message

You are not worrying unnecessarily. If there is anything you can do to get your sister in law to be more hygienic it would be public -spirited of you to do so.

I don't know how much influence you have with her.

She obviously respects ethnic customs more than social hygiene.

I don't have siblings, but from friends who have siblings who have died, I believe it is harder than losing parents, because it is an expectation that you will survive them. Your daughter in law is doing everything she can to mourn for her sister in the way she would have wished, whilst having a very short space of time to prepare for very different funeral rites.
That said, her husband is right to be concerned about her risk of exposure to covid through large unchecked gatherings displaying a foolish disregard for the even more stringent rules now.
She must go into strict quarantine immediately, but how she isolates from her husband and children I don't know.

Thank you all for your thoughtful responses. I feel a large part of the problem is the speed with which the funeral took place. In our culture the delay between the death and funeral is generally the period of grieving, we almost welcome the funeral when it arrives with its final outpouring of grief, and then the wake when people share memories of the deceased, often with a lot of humour involved. This is almost a cathartic experience, by the end of which we feel we can begin to heal. With a death and a funeral within three days - and a very very different type of funeral that we cannot hope to understand, I feel this has compounded the issues, not helped of course by a pandemic. I keep reminding my son that everyone is so very different and no-one has the right to say when grieving should end, it ends when it ends. In the meanwhile, I think he and the children are feeling pushed aside in favour of someone elses family. Life can be so complicated...

As MOnica says , it’s done now and I think with the religious aspect involved you would do well to keep out of it.

She has lost her sister so probably won’t listen to anyone at the moment anyway and just wants to be with her family.

If you are worried about her and your GCs and DS passing anything on to you, then ask them to stay away for the required period.

Bereavement of such a close family member is one of those areas where there is no one answer. To properly grieve is so important for the weeks and months ahead for your DIL so I think you have to let this go.

Clearly there are risks in all that she's doing.
I expect it's a feeling of 'last straw' that's made her abandon all thought of covid restrictions, in the face of her sister's death.

You are definitely not worrying needlessly.
It’s very difficult when you are unable to do anything about it
Take care of yourself & do what is right & right for you best wishes .

She is no doubt grieving and unfortunately sometimes the pain and grief makes us do and think things totally out of character.

The remaining people who we love most we tend to take for granted and our own grieving needs take precedence.

I think she is trying to cope in the best way for her. She maybe thinks life is so short, that the future looks uncertain, so that she is now just surviving in the moment.

I can see how potentially dangerous these gatherings are but I'd just grit my teeth as it will be over soon, I hope!

What is done is done, but she should now go into quarantine for the prescribed period.