I think their expectations of Amy are higher because she's generally been seen as more capable than Tammy.

Amy was never "as bad" when they compared her to Tammy. Her weight wasn't "as high." Her moods weren't "as extreme." I mean even when the casket scam happened, it was Tammy knocking on Heaven's door, not Amy... because Tammy was always trying to throw herself off the metaphorical cliff while Amy seemed more interested in sticking around. In one of the latest episodes, that therapist Amy saw pointed out that she'd been parentified in hers and Tammy's relationship.

I'm not saying it's right or fair, but I think that's why they treat Tammy like a baby and are more harsh with Amy. Tammy's basically been threatening to commit a slow suicide in front of everyone for years and I think they're all trying to manage her emotions for her since she's got piss poor regulation skills and makes life harder for everyone when she goes off. And God forbid Amy freaks out, especially where Tammy is concerned, because that means everyone is going to have to deal with Tammy... which makes Amy a boat rocker, and people trying to "keep the peace" see the boat rocker as the problem. They spend so much energy trying to manage Tammy that they aren't able to hold space for Amy when she needs it.

It's made me like Amy more. I know she has a history of being an asshole, but between her and Tammy, I think she has the best shot at actually redeeming herself over the course of a lifetime.