As previously reported, Heidi Montag's dad Bill Montag was arrested last week for allegedly sexually abusing a young girl for years -- find out what the Hills star thinks about the situation Credit:

Heidi Montag is sticking by her father's side. One week after Bill Montag was arrested for allegedly sexually abusing a young girl for years, an insider tells Us Weekly that the former Hills star is certain that her dad is completely innocent.

"It's all coming from an estranged family member with ulterior motives," the source exclusively tells Us. "Heidi doesn't believe any of it."

"She grew up in that house and knows her father's character," the source adds, "and 100 percent believes all of the accusations are false."

As previously reported, Bill Montag was taken into custody late last month in Aspen, Colorado for sexual abuse and aggravated incest claims. He allegedly was in contact with a 13-year-old girl (who has yet to be identified) from 1993 to 1998.

In court documents obtained by TMZ, Montag apparently "groomed her with back rubs" and performed oral sex on her. A source close to Heidi, 28, and her husband Spencer Pratt, 31, told the site that the accusations came from a woman "determined to destroy the family." (Bill officiated the couple's wedding in 2009.)

Montag split from Heidi's mother Darlene Egelhoff in 1990 and later married Terri O'Hara. In December, Heidi's sister Holly, 31, revealed that she's expecting her first child with husband Richie Wilson.