I think of Giselle as a tragic heroine, created only to suffer.
TBH, if Giselle was not written with such shitty setting she would be perfect waifu material, beautiful, caring, talented, possessive ( this can be a bonus or a minus for diffirent person ).
I can't really blame Giselle, she loved Abel all her life, her parents and the clan supported her love, from her POV her marriage with Abel is just natural and set in stone. To Giselle, Abel is not only her brother but a husband already so how she reacted is understandable, we can think of it as a wife feel like she was cheated on and the husband run away for no good reason ( atleast his reasoning is kinda hard to understand for the marren clan )
Well, I like her, I like Mea too, Abel should just be a man and married both. At this point law and order is trivial.
Or what, let Giselle forget about Abel and married some random guy ?
Think about it, in a similar case, if you had a childhood friend who is beautiful and you two are very close, you love her from when you both were young, growing up together, the parents accepted your love and all the neighborhood give you blessing for your marriage when you two come of age. Suddenly, the girl realize she and you can't married and run away from home to go far away never to be seen, what would you feel like ? Would you just smile and forget about your love and find a new one like nothing happen ? A human heart is not that easy.
I would say this is pretty shitty.
The one who hate Giselle is kinda superficial, they all hate her just because "muh-incest" , " don't stick your dick in crazy ", " jealousy much ",....
If you really consider her situation, she was a tragic character, she was not a bad character at all, her situation is just shitty.