I echo all above. Can I add that accessing support is not impossible.
I would begin with Macmillan as thy are the biggest charity, with the widest reach. Go on to their website, get a contact and tell them exactly what you have said. They are used to families doing this - your DiL needs time to process it all.
Of course, you my not feel you need it, but it is there if you do.
I would especially look up how best to deal with the children, get some things to say and do organised in your own mind. They will need you to be straight with them when their parents are too upset to cope.
NHS U turn on trans terminology