Drilling a hole in Aluminum Rib; how close?

I'm trying to align the holes in the leading edge sheetmetal with the
existing holes in the ribs. Sometimes they line up, sometime I can see
the hole in the rib, maybe 1/2 screw shank thickness away (in which
case I use a Dremel bit to enlarge the hole in the sheetmetal in the
direction of the rib hole) and in some cases I'm more than a screw
shank's distance away, so I'm going to have to drill another hole in
the rib. Question is how close can screw holes in ribs be before I've
weakened the rib?
If this were wood I'd enlarge the hole, glue in a dowel and redrill
the hole. What's the equivalent when drilling holes in ribs? - MikeH