Adam was the eldest child in the close-knit family. Jamie-Lynn, 33, also has a 38-year-old brother, Brian Sigler.
"Adam loved being an uncle to Jamie and Brian's kids," the actress' friend told E! News. "They gave him such joy."
Jamie-Lynn and her fiancé, baseball player Cutter Dykstra, are parents to son Beau, 1.
The actress has occasionally shared sweet family moments on her Instagram page. Days after she gave birth to Beau, her first child, she posted a screenshot of a touching iPhone text conversation she had with Adam.
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