I'm kind of lucky and have lots of nephews and my brother and sister are both younger than me.
Friends and loves who are estranged from families are in a difficult position. I have a couple of friends with law degrees who teach and are interested in medical law. The difficulty (in the UK, at least) is that if a person is unable to care for themselves, then the medical staff consult with the next of kin.
For unmarried people, that is likely to be the closest blood relative which can be a big problem if the family is estranged.
I'm okay - my family are people I get on with and also they are people who I know can deal sensibly with the end of life stuff if they need to. They are good at working together, supporting each other and helping with tough decisions (we've been through all of that with Dad and I have lots of confidence in them).
Serious illness in a partner with a rubbish family is one of the few things that would make me consider getting married. I'm very much opposed to marriage but it is probably the easiest way to become somebody's next of kin.