I think she was sad bc she saw oscar as a necessary piece for revenge. The sister has feelings for him & would gain a lot of power by marrying him / she would be (almost) at top of the hierarchy. So the fl wanted oscar to hurt the sister emotionally & if possible marry oscar and gain the power to revenge on her family.
As for Sarah, first she was truly just an investment for the fl, but then she wholeheartedly wants sarah to be happy (she states sth like that, when she visits sarah & her husband, I think). So, it truly was a process.
Therefore I don't think she is a tsundere, but rather calculative. And since she assured that having good feelings for sarah won't hurt herself in anyway (since sarah is too kind), she let her feelings be.
And oscar exchanging rings with the sister would lead that once again she would stand above fl, those fl will die in the hands of her family again.
I love the way you described this. I personally wouldn't of been able to describe it in such detail, you literally took the words from my brain I wished to convey and posted it here for me instead haha.
Why so confusing darling? Aria dear, I thought you'll become too invested in revenge that emotions which shows in your inner dialogues but then your reactions are sometimes contradictory. With how you lose composure with the exchanging of rings between Mielle and Oscar. Never thought you'll react that intensely. Seeing it I was certain you have feelings for him yet you keep on denying it. Also to Sarah, you keep on saying investment and whatnots but then later on you were obviously affectionate to her. You are beyond a tsundere, even your thoughts were opposite of what you feel. Anyways, I'll keep tabs on this manhwa. ε=ε=(ノ≧∇≦)ノ