Carrying weight

So I'm a first time mom and I babysit a 1 year old and a 9 month old. I was told by family that I should stop because I'm not allowed to lift to much weight. The 1 year old is around 30-35 pounds I believe the 9 months old I'm not sure but I sometimes have to carry her in her car seat. Do I need to be more careful or is it okay? I feel like I see moms do it all the time.