"You have six grandfathers."
Translation:Tá seisear seanaithreacha agaibh.
Singular nominative after Cardinal numbers (and cardinal numbers are adjectives in a phrase like "six grandfathers").
"Seisear" is a noun, not an adjective, and is not a number, per se - it's a noun, and is closer in function to "sextet" than the number six, (though "seisreád" is the Irish for "sextet"), and takes the genitive because it's essentially "a seisear of grandfathers".
It would be technically correct, so no reason. Apart from the fact that it is not a usual way of translating Grandfather, but http://www.teanglann.ie/ga/fb/athair has it, so just because the course designers did not think of it. Have you reported it? (As such changes do not usually occur from the forum but from reports)
And they might as well then add "seaisear aithreacha críonna" too...
If you're talking to more than one person, you say Tá X agaibh, if you're only talking to one person, you say Tá X agat.
Normally, a single person only has 2 grandfathers, but throw a couple of divorces into the mix, and someone could have 6 grandfathers, so both tá seisear seanaithreacha agat and tá seisear seanaithreacha agaibh are both valid, though you are more likely to be talking to 3 people when you are talking about 6 grandfathers, so tá seisear seanaithreacha agaibh is more likely.