i'm with A or B, too. B's age would not at all be a factor for me since i'm 42 with a 5 yr old and a almost 3 yr old. my parents were both 38 when i was born and i don't seem to have suffered any great harm from them taking me to college when they were 56. 60 would have been fine as well. they are in their 80s now and my dad is not in good health, but my mom is in pretty good shape. dh's dad was in great shape, but died in his sleep at the age of 63. obviously you never know when somebody's going to die which is why you are doing this, but your dad's age would not be a deciding factor for me. i think your sister might be a good choice, too, but maybe you'd like to wait until she had kids to see how that would change her lifestyle (quit work, keep working and put kids in daycare, etc). i would not choose C. as far as the "divorce" factor, well your dh married you didn't he? and your sister presumably had the same upbringing as you so i don't think that should be a factor at all.
hth
our choices aren't great. dh's brother isn't stable, his sister's health is poor and she has one special needs child and one grown son with health issues. my brother and SIL live in the same town we do, but he gives me all sorts of crappy parenting advice and doesn't demonstrate patience and gentleness. they have no children and are older than we are (50s). my sister and BIL (also older, but late 40s) have two teenagers and are pretty mainstream, but gentle, very soccer mom, but they live about 5 hrs away. i think we would pick them, but it's hard. dh and i have talked about it, but never formalized it. it would be hard to uproot dds from their town and friends, but i'm just not sure that my brother would be a good fit for them. of course, we hope it never comes down to any of it, but there you go.