Rowan and Alexia have separate corruption stats, and to get deeper into the twins’ NTR routes, Alexia’s corruption stat needs to be raised; Rowan’s corruption is irrelevant to those routes.
To raise Alexia’s corruption, the easiest thing is to pursue the early twin NTR events that don’t require medium corruption and have her work in the Tavern, as a maid, and in the breeding pits. Make choices that lead to her witnessing sex or cruelty, as well as choices that lead to her having sexual encounters (even if it isn’t actually sex). I think getting her caught when she helps Helayna escape, assuming you take that path, can also raise her corruption.
Once you witness the medium corruption event (you can’t miss it, it’s a nonrandom event that occurs immediately the week after Alexia achieves medium corruption and conspicuously features a tree), assuming you have a high enough relationship stat for Alexia with the twins, you’ll have the option to begin progressing further down the twins’ NTR paths. Which one depends on which you have a high enough relationship stat with; if you have a high enough stat with both, you’ll get an event that lets you choose one.