One thing that the SS Omega anime has made very clear is that i you are related to someone, you usually share at least one trait of them, whether is eye color, hair color or facial expressions, there is something that lets you know that yes they are actually family
Lets start with Ryuho
Looks exactly like his mom and even has some traits of his dad.
With Souma and his dad there is really no question here either
Haruto and his dad (same eye color)
Eden and Medea (Same hair color)
Even Sonia who is Eden is "half sister" still has the same eye color and facial similarities as him
Kouga and Aria?
-Different hair color
-Different Eye Color
-Different Facial Expressions
-Different Hair types
-Aria has really pale skin while kouga looks more tanned
In conclusion: They look nothing alike
No similarities whatsoever that would point out to them being related by blood