ummm, this might be a guy thing, but....

What did he do wrong?

Don't get me wrong, Lynne, you've got a right to be mad, just make sure you're mad at the right person, and IMO, its not him.

Your questions?
a) - Yes, carry on regardless. Not seeing him is just what his sister wants, so why give it to her? It just reinforces her behaviour.
b) - Yes, just don't be mad at him.
c) - Yes, to both parts.

P.S. your situation in general sounds eerily familiar. Just take out the vindictive sister, and it could be me... And I married the girl.

There's only one corner of the universe you can be certain of improving, and that's your own self. So you have to begin there, not outside, not on other people. That comes afterwards, when you have worked on your own corner.