Here is all we know:
(1) G1 has the two biggest values, G3 has the two smallest values
(2) Each value is different
(3) There are exactly 3 siblings
B>D
D> each of the three siblings- from this, we can infer that siblings can only be three of A, C, E, F
Since E is not one of the siblings, the three siblings are A,C,F
These are the relationships we know now:
B>D>A,C,F
These are the inferences we can understand from all of this:
Now, B can only be in G1 because it has to be one of the two oldest
A,C,F can not be in group 1 because there are already two people older than D
D can not be in G3 because there are three younger
So, D can be in group 2 (if E is older than D) but it can never be in group 3
E can be in any group because we can't infer anything about its relative age with others