You should definitely tell the little girl about the pg. If you dont her mother will be able to say "See, they didnt even tell you". Also, by not telling her, its like its not a good thing and you didnt think to share it with her. Ring her and tell her how great it is and what a great big sister she is going to be, how you hope the baby looks just like her, etc. If her mum is in her ear, all you can do is try and counteract it with love and patience.