Summertime Saga absolutely, though the sister is a little more verbally aggressive but at the same time slightly more slutty too. Insexual Awakening is the opposite, the little sister is super sweet and innocent with an underlaying naughty side, the older sister is nice but abuses the PCs trust to do naughty things, so I wouldn't say Insexual Awakening would be a good similarity in 'chemistry' with the PC. There is an actual romantic chemistry to A Town Uncovered's sister, with some relatable sibling fits and verbal abuse but in the end they still love each other.
I agree, ATU's sister has the best chemistry yet for sure, dare I say it's one of the best chemistries on Ren'py VNs