It's the two Jones brothers' boat
Take 'child' and 'children', and 'two brothers' : ask who owns the object and it goes straight after that.
the child does, so it's the child's thingy
the children do (irregular plural, formed without adding an 's') so it's the children's thingy
the two brothers do (regular plural formed by adding an 's') so it's the brothers' thingy
When the word already ends in an 's' as with 'Dennis' and "Jones", there are two ways:
Dennis' boat (pronounced Dennis's)
It belongs to the Jones's
It belongs to the Jones'
I prefer the full 's. A moderator will give you a 'ruling' on the 'correctiness' of these two.