Amanda didn't tell Fran about her relationship with her parents because; First, she's afraid Fran will hate her because she still thinks they would still be alive if she just trained them enough. She always think it was her fault that they died. Second, Fran actually had fond memories of her mother so she took a step back on proclaiming she raised her parents to kind of preserve that. And third, she don't want to tie her up to her parent's "origin" like how she tried to tie her parents up to the orphanage. That's why she didn't stop her when she said she was leaving instead she tried to go with her.
Her reasoning may be flawed and self-serving, but well, aren't we all the same on some circumtances?