It appears this thread, despite the title, substantially morphed into being about how to thwart Sis from getting an undue share of Mom's trust money as opposed to "Need suggestions for stable income."
With a pension, SS (amount not mentioned yet) and a $2.5MM portfolio, coming up with $100k/yr inflation adjusted income for mom should be a no-brainer. Any pre-occupation with not spending principle is silly. At mom's age, a 4% WR is more than conservative enough even if she lives past 100. There is no need whatsoever to focus on keeping principle intact. In fact, mom's WR will be closer to 3% virtually ensuring eternal sustainability (if the future reflects the past, etc.).
The only tough part should be telling mom there will be no lump sum withdrawal, for whatever reason, and getting her accustomed to receiving $100k/12 every month and living on it.
Poor mom! Only $100k/year!