When babies are ill, they normally only want to be with the person that they are familiar with
. The more often your sister comes round and plays with
the baby, the baby will form a bond with your sister, so in the future should you start to trust your sister again, the baby will be more comfortable with your sister.
There are lots of reasons why a baby cries and it may have been wind, a wet or soiled nappy, being overtired, being ill, or feeling not secure with your sister.