As a suggestion have you brought to her attention that she might be on the path to causing herself a lot of stress trying to handle the new job and her mother at the same time?
She appears to be one of those alcoholics that one of their biggest crimes so to speak is that they don't think through what they are actually able to handle, and over do it. She just doesn't get it--that stress of these types especially combined causes her to relapse.
So what can you do for yourself? If you see a train wreck on the way, get out of the way. A vacation somewhere for you? An old fishing buddy calls you up for a trip? I'm thinking solely on the short-term here.
Would her mother be insulted if you called her up directly and said that you don't know if your wife can handle the new job and company at the same time? I'd only suggest if your MIL and you were on very good footing and on the same team when it comes to your wife. I know...don't mess in her business, right?
Your wife though, from what you wrote, appears oblivious to just how much stress she can handle at any given moment. Some people don't see their own limitations as they bulldoze their way through life. They don't know how to go gentle on themselves.
I know I just wrote a codie response. A rational talk-through it and find a way to evade this response, and I know I will be told to stop expecting logic will work....but my brain doesn't yet work that way where I can discard finding logical solutions.