We know she drank at Christmas. No other time that we aware of. As I said ex's family let her brother drink and that's why DP asked. I guess that must have been occasions such as Christmas etc.
If we could prove it, we'd be on it. We can't.
DD isn't allowed to meet anyone other than DP and only for 2 hours a week according to the rules ex has put in place. She wouldn't know her grandparents etc she's said now we're expecting a child that DP has his own child and so can leave 'hers' alone' 
In one way ex has put DD at the centre of her universe, spoils her, controls everything in her life completely. On the other hand she says they're best friends and treats her far older than she is.