bone marrow donor...what to expect

My fiance is the only possible donor for his sibling just diagnosed with non hodgkins lymphoma. Hopefully he is a match but I am concerned about a few things. I have read a little(all I could find so far) about how after he donates he could get really depressed and blame himself if it doesnt work. We are only in our mid 20s so this is really scary for everyone. Also his sibling is saying "your gonna save my life", "Im so glad you were born", which im sure is normal but Im really worried about my fiances mental health if everything doesnt go as planned. He might be a match but we dont know yet. Ofcourse I am also concerned about how much this will cost us, will we be in debt and what are the risk factors for him (my google search doesnt help with this info). Another concern is how much of our life together going to change, being engaged, etc. I feel guilty thinking about getting married and having kids which his sibling clearly cant. How much of my life can I expect to change of be altered. See, I feel like a selfish jerk thinking about this. Does anyone have a similar experience? How do you cope? I feel like Im going crazy.