Brittney could very well have been suffering from "Stockholm Syndrome".
https://rainn.org/get-information/effects-of-sexual-assault/stockholm-syndrome
She might have wanted to meet with Donny to reassure him that she would not tell their "secret".
Chessie believed that Donnie had abused Brittney when she was younger, right?
So when it is said that Donnie was her favorite uncle, Stockholm is what comes to mind.
How could anyone be a favorite after sexual abuse otherwise?
If what I read on the other forum is true about Wendy being with Donnie that night, its not too far of a stretch to believe that even if Brittney reassured them that she would not testify against him, Wendy may have convinced Donnie not to believe her and kill her...or could she have done it herself with Brittney's own gun after a struggle. Were Wendy's hands and body checked for wounds or injuries?
It is equally easy to surmise that Donnie, coming to the realization of what they had done to attempt suicide, and Wendy cleaning his phone of incriminating evidence before she went back home.