12 month old - no babbling

[MarthaGurney]

We have a 3 yo dd and 2 (nearly) 12-mo dd twins. The twins couldn't be more different! One is quite advanced, nearly walking, 3 words, crawling, lots of babbling. The other twin is the opposite, and I'm a bit worried about her: she bum-shuffles (but no crawling forwards - she can sort-of crawl backwards, and is getting better at tummy time, so I think she'll be crawling properly in the next month), and doesn't babble at all. Like, nothing - no consonants at all. She makes lots of open-mouth sounds, can blow raspberries, kiss, purse her lips, but doesn't make any closed mouth articulate sounds (no 'mama', 'dadada', 'bababa' etc). In other respects, I'd say she was at the slow end of development, but still within a normal range: she can drink out of a bottle & beaker herself, feed herself (messily) with a spoon, has good fine motor skills (can pick up tiny things), is really sociable, laughs and smiles a lot, good on eye contact, claps and waves on command (so I'd be surprised if she has hearing problems). But zero babbling. There's nothing on the internet that says this is normal or OK. Does anyone have either any reassuring stories about children who babbled very very late and turned out to be absolutely OK, or, failing that, any suggestions for what might be wrong? (We're waiting for a referral to a speech therapist/ paediatrician, but in the meantime I'm stressing out.)