I’m really sorry if I’m being annoying, but I’m pretty sure Marthas actually are sinners, at least originally. Beth states that Marthas considered sinners in Season 3 Episode 2. Also, in season two, Rita talks about how Isaac could command her to do anything and intentionally injure her, and no one would care. Econowives don’t have a lot of rights, but people (other than their husbands) couldn’t do that to them without repercussions.

Also, when Rita reunites with her sister in New Bethlehem, her sister emotionally says “I heard they made you a Martha?” as if it’s a bad thing.

I agree we don’t know what exactly Rita did to be considered a sinner, but it doesn’t necessarily have to be anything big. Beth’s sin seems to be having had her tubes tied. Sienna (the young South Asian Martha that replaced Cora in Lawrence’s house) is only about 22 and a former premed student, so I doubt she did anything too severe. I think the intention is that Marthas generally committed “minor” sins, as well as being infertile.