How To Deal With Sexual Offending Behavior In A 4 Year Old?

My four year old daughter was taking a bath tonight when I saw her take her bath toy and try to stick it up her vagina. I asked her "what are you doing Mariah?" When she replied she said "daddy do it" I looked at her and said "daddy did that to you" and then she said "No, Grandma." A couple minutes went by an d she did it again but this time said her oldest sister India did that. I left it at that and told her that it was not appropriate. My daughter spends every other weekend and some week nights with her Dad at her Grandmas hours and her sister is also there. It makes sence to me that she might have seen her sister do that but I'm concerned since when she first did it she intially said "daddy do that." My daughter is behind in her speech but completely understands everything so its harder for me to communicate with her about this.