Ok next try.
We have three possible combinations, GB, BG, BB.
The chances of the boy being born on a Tuesday on each of the first two cases is 1/7.
In the BB case, we have 49 possible combinations, in 13 of which there is at least on Tuesday. So the chances of at least on boy being born on a Tuesday in a BB case are 13/49.
Which makes the overall probability....
...13/27?
Oh how I wish I remembered how to calculate these things...