First off i'd like to state that ANIME is NOT necessarily cannon.
No where in the manga has it ever been said that the 1st and 2nd are brothers. EVER
and the 1st DID have kids. Because Tsunade is his GRAND DAUGHTER. I read someone said that the 1st didnt have kids...
anyway another note is that in Kekkai Genkai families, NOT everyone develops the actual kekkai genkai, it's in their genes but if you guys have ever taken classes on this kind of stuff and DNA u'd know that some genes simply remain dormant.
For example families who have a lot of twins. Some of the family members simply dont get a pair of twins but their children may, or their children's children. Because it's in their family genes to have a high chance for twins.
Thus, it makes sense for the 1st to have Mokuton while Tsunade doesnt. But if Tsunade has kids they have a chance to develop Mokuton if it's a kekkai genkai.
Oh and by the way. I was gone this past weekend cuz i was camping. Missed me?