I started to say that you shouldn't donate out of guilt, but then I realized that guilt probably motivated me to some degree, at least at first. I donated to my boyfriend, who has Polycystic Kidney Disease, so he needed a transplant through no fault of his own. From his family there were no offers. My friend and my sister offered right off the bat...then my sister backed out early and my friend was the wrong blood type. I was scared about the whole thing, but I felt like I "should" get tested. I kept telling myself that I would just take it a step at a time and if after all the testing I was a match and able to donate, I'd decide then.
Right off the bat, I learned that I was type A and he was type O. The hospital said they were starting a paired donation program at some point, so that was OK, but it seemed like it might not happen right away. I was very ambivalent anyway, so delays were OK with me!
As I went through the process, I put a lot of thought and research into it. I scoured the Internet and read lots of donor blogs as well as medical studies, etc. I talked to people who knew people who donated, and to one person who donated directly. With each step of testing it was evident that I was very healthy and unlikely to be disqualified as a donor, so I had a decision to make.
Then they discovered I was a subtype of A...A2, which can often donate directly to type O, so we would not need a paired exchange. I started feeling like the stars were all aligned, at this point. As I thought it over and he got sicker and sicker (wasn't yet on dialysis) I realized that for me, the only reason I could come up with NOT to do it, was fear. And to me, that's not a good reason. So I did it.
Your situation is different. You have small children, I do not. I don't know how close you are to your family member, which would make a difference to me. I can understand your worry about her failing to take care of the kidney you give her (I told my boyfriend that was the one thing I asked of him...do his best to take care of it), I don't think that's harsh at all.
I'm not an altruistic donor like many on this site, I personally think I did it partially out of self-interest: to keep him around. I would not have done it solely out of guilt; in the end, I did it because I felt it was right thing to do and would make both my life and his life better.
IMO, you should think this through carefully and talk to lots of people.