"She’ll be a great mother because she is very tough, very strict," he told reporters at an event in Los Angeles, according to People magazine.
The singer announced last month that she was delaying the second leg of her "Unbreakable" world tour indefinitely to plan a family.
Although the singer, who turns 50 on Monday, hasn't said whether she's pregnant, her brother seems to think so.
"My sister Janet is my baby. She is fine but she's having a baby. A baby having a baby. We look at her like our little baby and now she's grown and having a baby," Jermaine, 61, gushed.
He added that the youngest of the famous Jackson clan is "feeling fine" and "loves to eat."
Jermaine said he cannot wait until he's an uncle again: "I can't wait to see how this baby is going to look."
Jermaine, who is the father of nine children, even offered his younger sister some parenting advice should she need it.
"I tell her, 'If you need any parenting skills or baby skills, let me know because I'm the best diaper changer there is on this planet, still to this day,'" he bragged.