Gracesmum spot on with the developmental thing - younger babies do not seem to really know who they know and who they don't in some senses. Then they do - and some are more wary of non-parents than others. At the same time separation anxiety when mum disappears kicks in and unless the substitute is one of her immediate "in crowd" then she will yell as loud as she can to try to summon her mother back to her side.
I agree that if you could spend a bit of time hanging out with the family group she might get the hang that you are one of her "in crowd" saljb.
While you are there, a trick with babes this age is to play "hard to get". If you can hang out at her house or a few hours, don't directly woo her. Play with something on your own maybe, something that will really interest her (a nice saucepan full of pegs or a bucket full of bubbles maybe - get in touch with your inner child and ignore, ignore, ignore), don't make eye contact or try to get her to join in. Just let her join in with your games at her own pace.
ALso play shy. Hide your face in your hands, peep out shyly and then retreat. That can sucker em in.
Another thought - while in her house, and while you are being happy together, practice mum going upstairs, mum going down the garden etc and see if you can build up tolerance of you being there and mum not.
Hope some of these ideas help.