"We were hoping she would own up to some of the things she did," Marshall told "Extra" in an exclusive interview.
Marshall -- who grew up very close to her sister but has been estranged since Hunter moved to L.A. 16 years ago -- thinks Hunter really hurt Edwards' wife, Elizabeth.
"They were married; she has cancer. She has two little children. She deserves some respect," Marshall said. "I would like to tell Mrs. Edwards that I feel so bad about the whole situation, and in my heart, I feel for her."
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Marshall admits that the entire family was "pretty upset" Rielle didn't apologize herself when Oprah gave her the chance.
"She just chose to take another path, and I don't know who she is," Marshall laments. "She's crossed over a boundary none of us would ever even think about."