I could see this happening, but why wouldn't Patsy confess? Yes, she caused the death of JB, but it was accidental and in the heat of the moment.
If she was scared or had been convinced to lie about it all, I also couldn't imagine staying with perv John all those years later. She would have been able to get a decent settlement, so fearing poverty would not have been a factor.
Patsy played softball in a womens' league- she may have had a very strong arm. Why not admit it? There are a few reasons why not, First, I believe BR may also have been molesting his sister and Patsy would not want any light shed on that. In an exam, evidence of molestation would have been seen. And also, JB may have gone into convulsions, coma and appeared to have been killed instantly (though she remained alive for a while).