User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-92.228.192.244-20180412060235/@comment-2602:306:CD9A:C860:7102:C4A1:E71B:392C-20180412161316

Sona being the sole successor to the Sitri clan has nothing to do with it. The truth is Sona just doesn't see Saji that way despite his qualities and only view him as a little brother. Plus Sona is aware there are other girls in her peerage who likes Saji. Also you have to take Sona and Rias personality into account. From the start Rias has wanted someone who liked her for who she is and coupled with Gremory's soft spot for caring deeply for their servants, it's only natural she would fall for Issei. Sona is not Rias and her personality is dofferent from Rias. To be honest I could see Rias fall for Saji if his role was reversed with Issei because of how similar Issei and Saji are.