Thanos' brother Eros aka Starfox looks nothing like the Mad Titan due to an Thanos' origin and connection to the Eternals has been a subject of debate ever since Thanos was retconned into being an Eternal decades ago.
As with many comic characters and stories, Thanos' backstory evolved over time. When he and Starfox first appeared in Marvel Comics in 1973, they weren't Eternals; in fact, their stories were rooted in Greek mythology, hence their names Thanos and Eros resembling - or in Starfox's case, being exactly the same as - the Greek figures Thanatos and Eros, respectively. Later, things changed and they became Eternals born to parents A'lars and Sui-San. However, Thanos required an additional change along with the retcon to justify why he and Eros looked so different from each other.
The Eternals' origin story in Marvel Comics is quite convoluted, with there being two sub-species of humans - Eternals and Deviants - as well as Eternals on both Earth and Titan, where Thanos and Starfox are from. The confusing part is that Thanos is affected by the Deviant Syndrome which gives him his signature purple look, despite not explicitly being a Deviant. And so, Thanos is very much an Eternal like his brother Eros. The difference is, Eros doesn't possess the same mutation, which is why he looks like the rest of the Eternals.
Eros and the rest of the Eternals look human because they're offshoots of Homo sapiens; Thanos, unfortunately, was afflicted with the Deviant Syndrome that transformed his look into something more akin to the Deviants and Deviant Skrulls (such as the indented lines on his face). Meanwhile, Eros and the Eternals maintained their human looks, albeit varying superpowers and godlike beauty. Still, despite Thanos and Eros' looks, they share the same bloodline and possess similar abilities, which is why Thanos is so strong and nigh-invincible.
To put the confusion to rest, Marvel Comics recently had Phastos confirm to Thanos that he is a true Eternal in Eternals #6, thereby ending the argument that Thanos wasn't technically an Eternal. Beyond their genetics, though, there's an interesting link between Thanos, Eros, and the rest of the main Eternals, as they're first cousins of Thena, who was born to A'lars' brother, Zuras. It'll be interesting to see that conversation play out on-screen at some point given the fact that Angelina Jolie plays Thena in the Eternals movie.
