June 20, 2011 04:00 PM

Gilbert Flores/Celebrity

When Laila Ali pictured the sibling bond between 2½-year-old son C.J. Muhammad and daughter Sydney J., 11 weeks, the mother-of-two is the first to admit she had her doubts.

Luckily, all of those worries have been put to rest.

“Right now he’s being a little brat because he wants to keep going on the slide, but he loves her,” the former boxer tells PEOPLE at the Elizabeth Glaser Pediatric AIDS Foundation‘s A Time for Heroes Picnic on Sunday.

“I was concerned that he was going to be jealous, but he’s not. He loves her to death, and he’s been nothing but a good big brother.”

With both of her children thriving, Ali has turned her attention to other important issues: shedding the pregnancy pounds and regaining her body after baby.

“I have 20 more pounds to go. People always say it’s easy for me because I’m an athlete, but I have to work. I have to eat right, I have to work out, and now I’m working out at home,” she says. “I have a spin bike, I have a treadmill. I’m running, I’m doing hills, I’m doing sprints … and 45 to 70 minutes of cardio.”

But while she’s seen some success, Ali — who is currently breastfeeding — is being forced to take her time.

“The hardest part for me is taking it slow. You can’t hit it too hard when you’re nursing, and I’m nursing,” she explains. “I could get it off much faster, but I have to take it slow while I’m nursing. The most I can lose is a pound a half to two pounds a week. I’d probably be doing double that if I weren’t nursing.”

— Anya Leon with reporting by Reagan Alexander

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