Kindly keep your other kids away from the infant, well there is no way to keep u away, so try to maintain the highest level of hygiene you can, wash your hands right before picking your baby and make him stay away from those rashes on you and other kids.
Since you are are just contracting the infection apparently you haven't passed the antibody to your baby hence keep him away from the infection as much as u can (if not too late already)
And in case your baby comes down with the infection, please DO NOT HESITATE TO VISIT A PEDIATRICIAN for optimal management

You have to try and separate her (your niece) to limit contact with other members of the family especially your baby.