Hello I'm doing the quest and when I try to convinc Slyvia to forgive her sister I can't change my dialogue choice no matter how much I reload.

There are three dialogue choices and I can make the first two, but the third one is locked to the choice I made the first time around. I do remember the first time that there was a third dialogue choice but now it just automatically continues the conversation as If I had chosen this dialogue choice.

You must go back to a save you made before you scotched the dialog the first time around -- that set a variable that locks the third choice. You must not talk about honesty or tell her you've heard enough, you must chose the true motives option. All this is assuming you have met all the other requirements for the happy ending, which are many. Always use an explicit save, do not rely on quicksaves.

I made a quicksave before talking to her, and when I reload it I can't get a third dialogue choice. Are you saying I need to reload a older save file?

You've probably pressed the quicksave button again inadvertently. Saved games, even quicksaves, do not change on their own. So yes, if there was a third option at first and there is not now, then your save has changed and you will have to go back to a prior one.