Had 4 figures in a group like so:

XZ
AB

X and Z are baddies, A and B are my figs. Can X attack B or vice versa?

X is Black Cat. Can she see B to affect rolls through Probability Control?

Line of fire cannot be drawn through occupied diagonal sqares so Black Cat could not use her PS through the diagonal. Addtionally the pieces chould attack each other through the diagonal but only with close combat attacks.

Mucho thanks!

You do not need LOF for Close Combat attacks. So, X could make a close combat attack on B (or Z on A). You do need LOF, though, for Probability Control, so Black Cat (B) could not PC X. She could PC Z or A.

From the Marvel Universe Rule Book (page 17):
A line of fire that passes between two adjacent characters, even on an exact diagonal, is blocked.

That's two now, Jr. You better pick up the pace! LOL