Well, I'd give your sister a call now and ask her what the problem is.
At first glance it seems perfectly fine to me, then when I think about it, if he'd just divorced his wife, and later started dating the sister, well, it does seem a little off. You generally steer clear of dating your best friends ex's, and your partners family.
But their reaction sounds more like they have a specific problem with the sister, or maybe the entire family and they think he had the chance to get away from them and is choosing the worst person to stay with.
Then theres also the issue of getting over his wife, maybe they think he hasn't gotten over her and getting with the sister is an unconcious attempt to still be with his dead wife, by staying with the thing closest to her in her sister. In which case they'll think he's simply not dealt with it and will eventually end up sad and unhappy with a woman he doesn't really love.
How long after his wife died did they start dating, its 2 years since they told the family, OP's known for a while, they could have been together without telling anyone since a week after she died. Finding comfort in the arms of someone else grieving type situation, its comftable and easy and helped them not be sad and not really deal with the loss.
Could be anything, still can't quite decide if theres anything wrong with it, theres nothing wrong in principle, but its a bit weird to marry one sister, then the other(assuming it got that far).