Buchanan said her title will be senior adviser to Tancredo's exploratory committee but will function as campaign manager until someone is appointed to that job.
Tancredo said last week he had not yet formally decided whether to run.
"I'm certainly encouraging it," Buchanan said. She said she expects Tancredo to formally announce his plans in the next week or so.
Tancredo, an outspoken opponent of illegal immigration, has described himself as an alternative for GOP voters unhappy with other Republicans. He has staked out conservative positions on social issues, opposing abortion rights, gay marriage and federal financing of embryonic stem cell research.
Tancredo has conceded he is an underdog, but Buchanan said his campaign was a serious bid for the presidency and not merely an attempt to force the hands of better-known GOP candidates on immigration.
"I believe Tom Tancredo can beat the top three and that's what our campaign plan is," Buchanan said, citing Arizona Sen. John McCain, former New York Mayor Rudy Giuliani and former Massachusetts Gov. Mitt Romney.
Last week, Tancredo spokesman Alan Moore said Tancredo's next campaign finance report will show he has raised more than $1 million in donations.
Buchanan managed the three presidential campaigns of her brother, Pat Buchanan. She is chairwoman of Team America, a political action group Tancredo founded to advocate for tougher border security.
A CNN spokeswoman did not immediately return a telephone call for comment.