You now have management over their memorial so you may add anything you would like. If your sister is buried in that cemetery you may post a memorial for her yourself. There is a way to post a non-buried cremation but I have never done it so you would have to look up the procedure.
You may post any picture or flower on any memorial yourself, because you are a member. For additional information or obituaries you would have to contact the person that posted the memorial. If it is a relative you may ask that person to transfer management of the memorial to you. Please always submit your Find A Grave Membership no. when doing so.
I'm always glad to help out. Best of luck on your research and once again WELCOME to Find A Grave.