I would remove the brisket from the marinade and transfer it to an air-tight large freezer bag (or something similar) with a flavor-neutral oil such as grapeseed if you can't cook it right now.
The level of acidity in that particular marinade is going to do two things to the brisket:
- Interfere with the natural enzyme production that normally happens in wet aging, which tenderizes the meat
- Cause the protein bonds in the meat to actually tighten rather than relax. This forces moisture out, and toughens the beef - which could result in a rather dry and tough brisket.
I don't think 48 hours is too long, but you should be able to tell if the adverse effects have taken place - you'll have more than a bit less brisket and more than a bit more liquid than you started with. If that's the case, don't panic but:
- At least start it in a pressure cooker, if you have one. You can finish any way you like
- If you don't have a cooker, go really low, and really slow on the cooking. 325 F max, covered, for at least a few hours.
As Joel noted, it's not a food safety concern, but more of a 'don't wreck your brisket before you start cooking it' sort of thing :) Leaving it to rest out of the marinade for another 24 hours in an air-tight covering might actually be the best idea.