Tia thinks Cedric loves Lysia because in the past, he proposed to her and they were engaged. When they broke off the engagement Cedric didn't associate with other women so Tia thinks Cedric was truly in love with Lysia. That is why she is guilty because she thinks she stole Cedric and Lysia's chance to have their love story stolen twice. What she didn't know was Lysia is only just like a sister to Cedric both in the past and present
Yup! not sure if Cedric love Lysia at the future(I hope not). But In Tia's memory now at present, Cedric proposed to Lysia 5years after Lysia became the saintess. But at present Lysia is not a saintess yet. So the proposal is yet to happen. But not anymore.HAHA. Don't get confused alright? the future I'm talking about is the time when the crippled Tia lives.
Could be that she was the only one he trusted to be his wife. Since yes he loves her as a sister but its better then marrying a stranger especially one he has to share secrets that could endanger his land and people.
Also they already have trust and affection for each other even if they never fell in love with the other they still could be good friends
"The first and last woman I thought of wanting. Before , and after... even if the world turned upside down, it will always be the same". DAMNIT! That scares me. I thought for a moment Cedric have memories too!