Lindsay Lohan's half sister not invited to family Thanksgiving

An extra setting will not be set at the Lohan house this Thanksgiving.

Lindsay Lohan hit headlines last week when it was revealed that paternity tests proved her father Michael is also a father to a 17-year-old Montana girl named Ashley Kauffman.

But it seems the news of a long-lost sister has the Mean Girls actress feeling indifferent.

“Lindsay isn’t upset, she doesn’t care, and seems to have no desire to meet Ashley,” a source told FOX411’s Pop Tarts column.

When asked about her new-found sibling, the actress – who stars in the upcoming Lifetime film “Liz & Dick” – told Good Morning America that she did not have prior knowledge that she had a half sister.

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However, the family insider told us that Lindsay did know for at least the last few months. And while the actress isn’t too fussed about the issue either way, it has apparently had quite the profound impact Dina, who is still struggling with the news.

Kauffman was reportedly conceived during a relationship with Kristi Horn Kauffman while Michael was still married to Dina Lohan in the 90s, and was born between their daughter Ali, now 18, and son Cody, now 16.

A rep for Lohan declined to comment. Her father Michael, 52, did not respond to a request for comment either. However, last week Mr. Lohan told reporters that he is “one hundred percent happy” with the paternity results, adding that “we're all okay with it and will deal with it in the best way we can."

Initially he denied speculation that he had another daughter, and refused to provide child support.