I hear ya, and since I'm only the SIL (least tenured at that) I keep my mouth shut when it comes to her finances. I'm here to support my wife as she tries to manage it. Her older brother is the Chosen Child and both parents have always deferred to his input. Since their dad died 3 years ago his influence has only grown. The sister is almost as much as a holy roller as their mother so whenever their mom says "God instructed me" she clams up and bows her head in servitude.
The MIL did well when she sold the old ranch. For the first time in her life she was flush (the FIL was a financial idiot and unemployed by choice by his mid 40s). She bought the place she's in now w/ cash, paid for the renovations, and oh yeah, replaced her car twice inside of 2 years with newer (but smaller...yay) models. She should die with plenty of money in the bank which makes the reduced property taxes as a reason for moving ring hollow.
So thankfully, her money shouldn't be an issue. She's agreed on a price for her friend's place and it's less than what she should get for her place. The bigger concern is her moving farther away from the amenities a woman her age needs. The more I look at it, the more I see the fingerprints of her son influencing a lot of this. The ranch community has features that he exclusively would benefit from. He may be positioning himself for a great inheritance while my wife & his sister get the shaft. That was the what their father intended to happen originally, BTW. But after his death & the MIL's relocation her will was supposed to have been updated for a clean, 3-way split. I'll let the other BIL dig into that deeper (he has zero qualms about putting his nose into the finances).
Fun days ahead...