My company is planning to move my family and I to Zurich in the next six months. Coming with me will be my wife, 11-year old son, and 9-month old son. I understand that if I have a B-permit for working in Switzerland, they will all be able to get visas to come live with me. (My wife does not intend to work, as we are expecting again in September.)

The question is about my mother, who is retired and would like to visit us for a prolonged time on a regular basis. She spends the summers travelling the US in her RV, but would like to spend the winters in Zurich, living with us, ideally between 4-6 months a year. She would not work, either, instead just helping us with the kids. She is widowed and I am her only child, so she'd like to be able to spend a significant period of time with us, if possible.

If she is living with us and not working or drawing from the Swiss social security system, can she qualify for the same sort of visa my wife and children can? A tourist visa would allow her only 90 days out of 180, I understand, which would mean she'd have to travel twice a year to spend that much time with us.

She is not a "high net-worth individual", but has enough retirement and investments to be able to support herself, and she would continue to own property in the US.

Any advice on visa/tax possibilities for her?

All members in your family, especially in bloodline one up/down, have the right to live under your roof. Provided you have enough funds to support them. As she receives a pension, and I assume is more (or close) then a 1000.-, it should be not problem. The BfM could ask however an educate flat, dump rule heads plus one, so 2 plus (2)/3 kids plus mom = 7 room apartment, You can however argue that the baby sleeps with you and the two elder kids in one room. But a 5 room for sure.

Then she should get a B-Permit, same as your wife. You have to file for a family reunion visa to start with. Go to "Gemeinde" to initiate the process.

The 'search' function is your friend. This has come up several times before. I am not so sure it is as the previous poster wrote, especially for non-EU. See here.

Visitors visa for greater then 90 days (for parents)

You can grant anyone a "right" to live under your roof. However you'll find that doesn't lead to the automatic issuance of a residence permit as far as non-EU citizens are concerned.