Krista,
Your MIL must really feel fortunate to have you as part of her family. How amazing that you could potentially be her savior.
I am still very new to the kidney donor scene (I donated a kidney to my wife just this past week), but I can relate to the feelings you described. For me, it felt like I was suddenly marked as "the donor" as soon as I expressed willingness to donate, even when there were other better donors available -- even some perfect match family members. It almost felt like everyone let out a collective sigh of relief and quickly stepped aside to make way for me. I was very confused by it all. After talking to some of them, I came to realize that all their decisions were made based on their own independent, valid circumstances... maybe there were health concerns, job concerns, plans to start a family, etc. All those decisions were perfectly valid and personal, and I actually felt better after talking to them a bit. Just like them, your reasons to donate (or not) are completely personal and valid. Just be sure it's something that YOU want to do, regardless of who else could potentially be doing it.
For me, I decided that maybe I was placed into the donor role for a reason. Maybe my kidney was healthiest out of all the other candidates. It just took a great deal of faith to come to peace with it and go for it. You never know... maybe your 27-year-old brother-in-law hasn't taken great care of his kidneys over the years and yours is the best choice for her anyway.
Best wishes for your huge decision!