Ben Raanan said he is confident that his sister and his stepmother will return home.
He spoke to ABC7 Chicago via Zoom, and he is operating on the belief that they are alive.
Natalie Raanan, 17, and her mother, Judith, are among 13 Americans who are unaccounted for.
They are believed to have been taken hostage by Hamas.
The two had traveled to Israel to celebrate a relative's 85th birthday and the Jewish holiday season, their rabbi said.
SEE MORE: Evanston mother, daughter held hostage by Hamas in Gaza 'are alive,' relative says
The family hasn't heard from them since a week ago, after Hamas launched a surprise attack.
"We have received intelligence from the American government as well as the Israeli government that the belief is that they are still alive and somewhere in Gaza. However, we have not yet received any sort of proof of life from Hamas," Ben Raanan said.
Natalie Raanan recently graduated from Deerfield High School and was looking forward to taking a break and visiting family overseas.
Her bother said she loves art and fashion and that he was going to take her for her first tattoo, when she turned 18, which is in about a week.
"She's quite addicted to her phone, madly in love with fashion," Ben Raanan said. "She loves wearing these like kind of ridiculous nails that are like 6 inches long, which can't open doors with, but she thinks they look great."
He also said the family has spoken to President Joe Biden at length.
They feel he is committed to doing everything he can to bring their family home.
On Wednesday morning from Israel, Biden said America's top priority is bringing home American hostages.
"For those living in limbo, waiting to learn fate of a loved one, especially family of hostages, you're not alone. We're working with partners to bring home those captive by Hamas," he said.