NICK Jr.’s Dora is getting ready to explore the joys and pitfalls of big sisterhood.

The little-kids cable network is keeping the details a deep, dark secret, but the 7-year-old heroine of the hugely popular “Dora the Explorer” animated series will welcome either a little brother or little sister into her family next month, Nick Jr. announced yesterday.

The new episode, titled “Dora Big Sister,” is scheduled to premiere Monday, March 21, at 1:30 p.m.

“A lot of preschoolers will be able to identify with Dora’s sheer delight in becoming a big sister – she couldn’t be more excited to welcome the new baby,” said Brown Johnson, executive creative director for Nickelodeon Preschool Television.