Everyone grieves differently.
When your spouse dies, rather than when they up and leave you in the other sense, it's absolute and final. Particularly if you knew it was coming, the grieving process begins before they have even left.
In this sense, it's sometimes easier to move on (in terms of begining something new) after the death of a partner than it is after a break up. There's no lingering doubt about the right and wrong of it. There's no hope to get back together with the person who once occupied your whole heart. There's none of the baggage of a break up.
Think of it this way:
When two people break up it's like they were sharing a house. One person leaves a the house, bit not all of their stuff can be taken in one trip. The house has to be revisited, paths continue to cross.
When someone dies, you both leave the house. The house has been sold. You remember the relationship, but you don't go back. You don't cross paths.
People saying nine months isn't long enough for someone to move forward with their life have a very black and white idea of what grief is.
OP has also stated that there aren't children involved, so she won't be stepping on any toes.
This man deserves to be treated the same way as any other single male re-entering the dating scene after the end of his last relationship, nine months prior.
Your sister should meet up with him, should get to know him again and see where things go. It might go nowhere. It might go somewhere fantastic.