My niece is 11 months old and only says 2 words?

She only says 'mama' and 'dada', the rest is complete babble. She knows what yes and no mean, but can't say them. I'm pretty sure she knows what sleep, or 'bedtime', means but she also can't say those. Is this normal for an 11 month old? Should she be talking more? This is my sister and brother-in-law's first baby, and I'm just wondering if maybe they're messing up or doing something wrong. I haven't confronted them about it, but I just thought she'd be talking more by now...