Thanos’ brother Eros aka Starfox looks nothing like his brother thanks to an Eternals Deviant gene. The Eternals, as we know are an immortal race of cosmic beings who have been around in the Marvel universe for thousands of years but have rarely interfered in the events that plagued the Avengers; even when Thanos is involved.
Thanos’ backstory has evolved over time.
He and Starfox appeared in Marvel Comics in 1973 and they weren’t Eternals back then and their stories were rooted in Greek mythology and hence their names Thanos and Eros resemble the Greek figures Thanatos and Eros. They later became Eternals born to parents A’lars and Sui-San. But, Thanos got another change with the retcon to justify why he and Eros look so different from each other.
The Eternals’ origin in the Marvel comics is very complex and there are two sub-species of humans – Eternals and Deviants, as well as Eternals on both Earth and Titan, where Thanos and Starfox are from.
Now the confusing part is that Thanos is affected by the Deviant Syndrome which gives him his signature purple look, despite the fact he is not a Deviant. And Thanos is an Eternal just like his brother Eros. The difference is that Eros doesn’t possess the same mutation and that is why he looks like the rest of the Eternals.
Eros and other Eternals look like humans because they’re offshoots of Homo sapiens while Thanos was afflicted with the Deviant Syndrome that transformed his look into something more akin to Deviants and Deviant Skrulls (like the lines on his face). On the other hand, Eros and Eternals have maintained their human looks, with varying superpowers and godlike beauty.
And despite their appearances, Thanos and Eros share the same bloodline and have similar abilities, which is why Thanos is so strong and almost invincible.
Marvel Comics, recently put this confusion to rest with Phastos confirming to Thanos that he is a true Eternal in Eternals #6 and ending the argument that Thanos wasn’t technically an Eternal. There’s also a very interesting link between Thanos, Eros, and the rest of the “main” Eternals as they all are first cousins of Thena who was born to A’lars’ brother, Zuras.