Play was stopped for some minutes when Nico complained of suffering racial abuse from a section of the fans inside the Wanda Metropolitano.
When play finally resumed, Nico scored and celebrated by pointing to his skin, which led to some of the home fans booing him.
Although Athletic Bilbao ultimately lost the game 3-1, Inaki has shown support for his brother in the wake of the racial abuse.
“My brother does everything the best he can on the field. He enjoys making people who watch him happy and I have not understood why people criminalize him,” the 29-year-old said, as quoted by Citi Sports.
“I think that those whistles should be directed at the people who said those insults and not at the victim. No.”
He added: “It’s not the first time it has happened. In these type of things we have to build together not only in football but a good world in which these type of actions cannot be tolerated.
“I think this scourge has to end. I think that in the end, people have to understand that it is not easy to put up with this type of insult.”
This comes after Inaki also threw his support behind Vinicius Jnr in the wake of the Real Madrid star’s encounter with racists during a game against Valencia last year.