Okay. When an object collapses, it follows the path of least resistance.
The twin towers had a core of steel girders and an outer 'mesh' that gave them strength and flexibility. The mesh acted like a fly net, so that even if a few parts of it were broken (if, for example, a plane collided with the building) then the rest would remain strong and stable. The architects designed it so that the buildings would be able to take a few planes hitting before any major structural damage occurs.
Now, we know how the planes hit the towers, on one side or the other, but not all four at once. This means that any damage is localised at these locations, and even if the 'jet fuel weakened the steel beams' supporting the towers, which it's thoroughly unlikely to have done, the beams closest to the impact zone would be weaker than the ones on the other side of the tower.
The centre of the tower was the path of greatest resistance, so the beams - if they were weakened - would have bent/snapped and the top of the tower would have fallen sideways.
The lower floors - unaffected by the impacts and supported by the unweakened sections of steel and mesh - should have been just fine. There is literally no way physically that the entire thing could have just collapsed on itself because of the way it was designed. For it to happen once is an incredible anomaly, for it to happen twice in a few seconds is statistically and physically impossible.