Lindsay Lohan -- My New Sibling Will Be A ...

Lindsay Lohan and her dad might be in the middle of a nuclear flight, but LiLo's still gotta start her sisterly duties in three months ... and TMZ has learned the new baby is gonna be a BOY (phew).

Sources close to Michael Lohan tell TMZ he and his future baby mama, Kate Major, found out the good news just days before LiLo said she was cutting all ties with her father and vowed to never speak to him again ... (again).

We're told the couple is still brainstorming ideas for names ...