WMG / The Road to Cydonia

Ranma and ryoga are brothers

The road to cydonia is apart of the altered destinies universe and is similar to A Tale of Two Wallet. in that series ryoga’s mother was sent to seduce Genma Saotome so she can have his offspring. When that failed she used an illusion to make herself look like Nodoka and conceived Ryoga. Tetsuya is actually the father of Ryoga’s younger sister Ryomi only in the ROC universe Tetsuya stayed and Ryoga always saw him as his biological father. The biggest clue for this in the series is when the Nerima Wrecking Crew first joined Xcom and replaced their normal attire for uniform professional attire Ukyo noticed that this made Ranma and Ryoga look even more alike then usual.