This is a little tricky. My MIL is widowed and has Alzheimer's . She has lived in our home going on 3 years. Even though my wife works just to get out of the house for sanity, she still takes on the majority of the cargiver responsibility. We have weekday caregivers about 8 hours per day. Even before she moved in my in laws have been extremely co dependent on my wife. Vacations, dinners, outings, they've always insisted on being included, and would pout if they found out they weren't. So it has been overshawdowing to say the least. Well now my wife found out she has a brain tumor and wants to tell her mom, but will also need some separation for a while to recover and go through therapy after the surgery. Her dilemma is that her mom will be left without a clue if anything goes wrong, but if we tell her we feel the co dependency will Get worse, as her mom already can't even stand to be left with the rest of us for a few minutes, asking constantly like a child in the back seat..."when will she be back?" So we fear if we tell her, it will be that one thing she dwells on constantly, giving us no peace whatsoever. An example, my daughter borrowed a pair of my MIL (her grandmothers) earrings for one evening. My MIL could NOT stop thinking about my daughter losing one of the earrings, to the point that she got up SEVERAL times all hours of the night to come into our bedroom and ask if the earrings will be okay....so imagine telling her that her daughter has a brain tumor!!! She will insist on being by her side at all costs, Which will make surgery and recovery very difficult as my wife will need to focus on herself. Her mother is 82 and in very good physical health, which is even more taxing because it's like having a 5 foot tall toddler that gets into everything. So we can't be chasing her around the house keeping her out of things while my wife is trying to recover.....so...do we tell her? If so, how ? I say we don't, but the guilt of not telling her is causing my wife even more anxiety.
You also talk to MIL's doctor and get the proper medications prescribed to manage MIL's OCD/anxiety during the transition period. Avoid contact for two weeks while she settles in. Calling daily is counter productive.
In light of your wife's serious medical condition, I think I would make arrangements for your MIL to be cared for out of the house or have your wife recuperate out of the house, so you wife can get uninterrupted rest. From your description, I can't see how your wife could get any peace or rest in the house with her mother being there.
Is your FIL living there too? If so, can he help? I suspect that the key may be to take control of how MIL runs the house and take charge. After a certain stage, a person with dementia can't run the household. Establishing the lead is important.
I'm not saying that you have to tell MIL the details. What about saying that her daughter is having some surgery and needs time to get well and that she will be staying elsewhere while that happens. You don't have to be specific about what kind of surgery or the seriousness. Later, she will forget about it happening, so I wouldn't fret about details.
I think I would leave the recovery period open ended, since your wife may not be able to resume in home care of her mother. I'd support her to know that it might not be feasible. Her mother would adjust to something else. It happens all the time. I wish you all the best and I hope your wife has a fast recovery.
Maybe you could just say that your wife is having some elective surgery but you don't understand all the issues and leave it at that. MIL will need reassurance, and it's probably likely that something as complex as brain surgery won't be completely understood anyway.
I'm wondering also if you can find some out of the house activities, such as at a senior center, for the recovery period to give your wife some extra rest, even if MIL doesn't want to leave.
And given her reliance on your wife, it's likely I think that if she did know of the type of surgery, she'd be panicky not only for your wife but for herself.
I do hope the surgery is successful and your wife is able to make a full recovery.