[MadMax]

After looking at those pictures, I'd say that there is a stress concentration at the sharp corner on the tabs which isn't helping. One tab shows a curved fracture ending at opposite corners of the tabs while the other tab shows a clean break across the cross section of the tab. I'd say that the first tab failed due to fatigue which put all the load on the opposing tab which failed in a fairly fast fracture mode (straight clean break).

Tankdude is right on the money recommending a change to mild steel over 7xxx series aluminum. High grade forged aluminum billet has good strength in comparison to mild steels (roughly similar) but it does not have the fatigue resistance. Steels generally have much better fatigue performance to aluminum alloys and weight doesn't appear to be a factor with this part.

I'm seeing a lot of operations in making this part. A lot of thin sections that are difficult to fixture to for later operations. I get the feeling that this would be a good application for sand cast steel. Fill in the cross drilled holes and get the part sand cast. Finish drill the holes and tap them.