In addition to actually repairing the fender it is also essential to address the reason it cracked in the first place. In this case I'm willing to bet that when you installed the stainless steel brake lines you did not reinstall the two clips that retain the original lines just above the calipers. Without those clips when the bolts were tightened down the fender was pulled further out, stressing the plastic which failed at the weakest point.
All you need to do is insert washers the same thickness as the clips then do the repair.
If it has already been done, it is safe to assume it is possible to do it.
On the other hand, if it has not been done never assume it is impossible to do it.
------- Rob --------