It was only two months ago that Lindsay Lohan discovered the shocking truth that she had a 13-year-old half-sister after OK! broke the news that her dad, Michael Lohan, had fathered a child with another woman while he was separated from LiLo’s mother, Dina Lohan. And now, OK! has confirmed with sources close to the Lohan family patriarch that the Georgia Rule actress, 22, will soon be gaining yet another family member  — a stepmom.

"Michael recently proposed to his girlfriend Erin Muller," the source reveals to OK!. "But they’re keeping it quiet for the moment."

The pair may not be saying anything out loud, but the ring Erin was flashing last weekend at the Mercedez-Benz Bridgehampton Polo Tournament was screaming, "we’re engaged!"

<img src="" width="333" height="500" alt="Ramona Singer with daughter" />
(Close-up view of Erin’s ring; photo: Marion Curtis/Starpix)

Earlier this summer, the 48-year-old told the NY Observer about Erin, "She has a great family and I love her dad."

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