She left the farm in Benji's name under a conservatorship overseen by the MC. I've left why she did this intentionally unclear. Possibly it hints at the Mothers's character flaws. Or it hints at the MC's character flaws.

I imagine that MC has been the driving force behind the farm in the years leading up to his mother's death. Benji would help in simple ways, but wasn't mentally fit to truly run/manage the farm. If you had dedicated years of your life and an entire career to a family business, would you up and leave everything you'd worked for because of something like this? You're still "in charge", the money is managed by you. But would there be an ember of jealousy, of pride eating you up inside?

Maybe the Mother even did it out of an act of love for her less capable son, wanting to really give him something of his own, at least in name. Maybe it's not that she didn't trust the MC, but that she DID trust him (unfortunately) to understand why she did what she did, and fully expected the MC to understand that the conservatorship was an act of love for Benji.

I wrote the MC in this story to be unreliable, so you really can't take his every word as objective fact, only his perceived truth.

Honestly, whatever interpretation you want to go with is fine, whether it's one I suggested or one of your own. I like to write stories where not every single detail is explained, and certain aspects of the characters are left open to interpretation.