Is it normal for my baby to take less milk when she's with the sitter than when she's with me?
Yep. Some (but not all) babies prefer to "hold out" for mama, drinking a little less when you aren't around. As long as her weight gain and temperament are okay, this really isn't a big deal. She'll make up for it with extra (or extra-long) feedings when she's back in your arms.