i see the claire/peter thing being more as a sibling relationship, as it was like the older brother claire never had. even if they weren't related, i don't think they would be boyfriend/girlfriend.
and it does look like molly is the new tracking system. i think mohinder is also referring to the practice of having a second baby to save the first one. in alot of instances, when the first child had a bone marrow disease, they would have a second baby in order to make donor marrow. so in theory, mohninder would have saved his sister by being born and giving her the healthy antibodies.
molly, on the other hand, has no known siblings, so getting good antibodies will be tougher, so mohninder's blood could be rejected.
i see one of three things happening here:
1. mohninder's blood matches, nothing special.
2. it doesn't work, molly is sick.
3. mohinder is an evolved human.
and it is also reason to think the sister was an evolved human.
and sounds like thomson is in the company as well, with linderman and possibly nathan.
and i'm thinking sylar is a tragic hero of sorts...
this week's show was sort of a downer after last week, but still good.