The issue is that the husband has passed away so sponsoring the husband and including the step son as a dependent is no longer an option. The step son's mother is out of the picture and has been for years.She can’t sponsor him directly, but can sponsor the husband through spousal and the son will get it as dependent I believe. Problem is documentation. I’ve heard of a case where the spouse could not take her kid as a dependent because she couldn’t get authorization from the ex-husband that was only doing it out of spite.
There could be avenues through formal adoption but this is more complicated given the step son's mother is likely still alive (although location is unknown) and will also depend on the country. This is why consulting with a lawer is the best next step in my opinion.