"I couldn't tell you, and I don't even think at this point that he knows what he's going to do after the season," he said.
Chris also came to his younger brother's defense, stating criticism of the tight end throughout the 2018 season was based on a fantasy football, numbers-driven culture, not watching game tape.
"If you actually watched him play throughout the year, he was crushing people all year," he said.
Gronkowski finished the regular season with 47 receptions for 682 yards and three touchdowns. The catch and yardage numbers were the lowest in a season that he played at least 10 games since his rookie campaign of 2010, and the three scores matched the lowest mark of his career.
The 29-year-old University of Arizona product made just one grab in the Patriots' Divisional Round win over the Los Angeles Chargers, but he was a bigger focal point in the AFC Championship Game with seven grabs for 79 yards in the team's overtime win against the Kansas City Chiefs.
In December, the longtime New England superstar said his focus was on the rest of the season, not whether he was going to retire in the offseason.
"I haven't been thinking about that at all," Gronkowski told reporters.
But Ian Rapoport of NFL.com reported before the Chargers game that Gronk would take his time making a decision about whether to return for the 2019 season, just like he did for 2018. He noted that "some people close to him believe Gronkowski's decision will go the other way."
If he does retire, he'd be walking away from the final year of his six-year, $54 million contract, which comes with a $9 million base salary for 2019.