Hello Midwestgal and welcome!
As someone who is married to a person who recieved a transplant, I thought I would respond as well. I donated a kidney to my Husband in 2009. He had kidney problems since childhood, so I and the rest of the family always knew he would eventually need a transplant. However, that was always going to happen "someday". Well, when "someday" became "now", I can tell you that it still hit my Husband (as well as me) very hard. There was a lot of disbelief that this was happening to him. It was also very, very hard for him to ask any family members for a kidney. He was very fortunate in that he never really had to ask, his Parents, sibling and I all came forward and offered to be tested. (Both of our kids did too, but as they were ages 18 and 20 at the time, we wanted to avoid that if at all possible). In the end, I ended up being his donor. But I can tell you, it was a really hard thing for him to accept a kidney from anyone living. I know he had great concerns that it could negatively affect our health. Between him not feeling well (His kidney function was at about 12% at the time of his transplant) and having to deal with the fear of how a transplant would affect his own life, and having to worry about me going through surgery and having to live with one kidney, it was alot to deal with.
I am in no way trying to make excuses for your sister-in-laws behavior, or downplay what you are offering to do for her. You have been most generous to even come forward and offer, much less do through all the testing and commit to donating. I am just saying that maybe your sister-in-law is having a really hard time dealing with this, and maybe she is concerned for your well being or that you will change your mind. It is a hard thing for a recipient to get their hopes up, just in case things don't work out, for whatever reason. People really do react in some strange and unusual ways sometimes when dealing with something so stressful. Please know that in no way am I trying to talk you in to or out of donating. I just wanted to offer another persepctive of what she may be thinking/feeling and may help explain her actions.You will need to go with your heart on this one, and do what you feel is right for you.
I wish you all the best with this. Please do keep up updated as to what you decide. Please also feel free to post with any other questions or thoughts you may have. We are all here for you.
Linda