What are the kind of medical situations when a sibling would enjoy to 'give' something to hold the other alive

What are the kind of medical situations when a sibling would enjoy to 'give' something to hold the other alive
Blood - especially if it was a pink type (accident or operation may require it urgently)
Skin - if one sib was extensively adjectives the other might be able to donate some
kidney - dud is only principle for transplant
bone marrow - sib with cancer,
Siamese twins are a special luggage - they might give up their entire natural life, access to a limb, internal organs, schedule is almost endless if they are to be separated.
Organ transplant: Liver, kidney, bone marrow...
Kidney transplants, blood transfusions, bone marrow donations...