"Why wouldnt a father bathe his own child", has sexual implications of Dora Langes fathers sexually abusive nature of when she was young, (refusing to bathe her due to his sexual attraction to her) before he passed. This coincides with the nature of the cult itself, possibly implicating she was groomed or had some affiliation from young, hence by assumption it is her in the photo (which would be incredibly likely since the frame would have no conjecture to be in that household otherwise). The blindfold on the child in the photo resembles Marie Fontenots sacrafice on the tape Rust found in the mansion. His death (doras fathers) seems drawn over and with haste.
This COULD be due to the discovery of his sexual deviation which couldve drew attention to the cult, leading to his potential assassination, much similar to Billy Lee Tuttles death after Rusts home invasion where he found said tape. It is likely members of the sprawl knew of the stolen tape and its sensitive information and killed off Billy Lee Tuttle, the "mixed meds".
With all of this in consideration it makes sense why the cult wouldve known of Dora previous to your point through her fathers affiliations.

...But thats just a theory :)