that's the bad thing in almost most DXD fanfiction, they nerf rias like the original, because if she really wanted to get out of her commitment, she would train a lot to increase her strength, her brother could also assign her one of the members of her nobility to train her, and when the canon starts, she would have a very high power level, and she could make her nobility train harder too, because if she loses, everyone loses. It is very stupid to think that she would not do anything, the most logical thing is that she trains a lot and the members of her nobility the same, they solve their traumas, and in the ranking game, quality beats quantity, and if you rias you would be strong enough , abuses riser in the fight, which makes him not want her as his wife for being stronger than him and having hurt his pride, canceling the engagement.