My engagement got overshadowed rather quickly by my grandmother being diagnosed with Stage IV ovarian cancer (again.) This time, she was out of options. Everyone wanted us to hurry the hell up, get married and pop out a grandkid for her. We (politely) told them to go to hell, we weren't rearranging our whole damn life because an old bat I didn't particularly like (and still don't, even if she is dead) was about to croak. We got married on our time table and she croaked 4 months later.
I'd say ignore your sister, her wedding, and her male unless required for frosty politeness and family politics. Better for all involved.