Hrm...most of those are just coincidental.
-Main characters are farmers who refuse to take part in a war that doesnt directly involve them, even though both wars did indirectly involve them.
Farmers who don't want to get involved in wars but have to is a very common plot theme.
-Both characters have a deceased wife.
Another common plot
-Both immediatly take part as soon as there sons are effected by the invading force.
What do you expect?
-Several family members are killed in a short time.
When there is war and a family...death usually happens to make us emotionally involved
they both lost two sons, and a daughter in law.
Now that is pretty strange...
Both of the wives that died, were named Ann.
That's really strange...
They both raided a convoy, the wagon train in the Patriot, the train in Shenandoah, to free the son that was taken prisonor.
That's kind of similar but not really.
I would imagine that Gibson watched this movie and was certainly influenced by it. As well he should be. But it was not a complete rip off or anything.