Guardian/Adoption of my sisters kids?

My sister passed away on March 26, 2023. Now her husband has been diagnosed with CKD stage 4. The kids are 15 and 13. I was wondering if it is possible to already start a process if i have to be their guardian/foster parent. If something happens to the father suddenyly then it can take too long for the legal process. They currently live with mum who is also 77. Given the situation, the kids can be left all alone. What are the possibilities for me in such a case?