Next of kin, relationship and who makes decisions

[WartOrNot]

If a person is too ill to advocate for themselves, or dies, who makes decisions for or about them - their partner of many years or their elderly parents? What about their siblings? There are no children.

If not married then parents unless they have have say specified with the hospital who their next or kin is but I think legally if say pulling life support it would still fall to parents if not married ?

With any luck they have done a Health and Welfare Power of Attorney in advance.