Help!!!

Something was brought to my attention today and I just can't stop thinking about it and I really need some opinions and input on this. For the past few months my grandma has been taking my daughter (2) a few days out of the week. (I work full time) My family is really great and we're all pretty close so she takes my daughter over to my aunts to spend time with my two younger cousins also girls (9 &12) So today we're in the car and my daughter is in the back seat with the 12 year old and she starts hysterically screaming and crying in pain, Obviously I couldn't see what happened so I asked her and my cousin interrupts my daughter and says it must have been a toy she was playing with so Im like okay.. yeah... Didn't think much of it until I brought my daughter in the house and asked her what happened and she tells me her cousin hurt her! So I asked her where and to show me and she points to a spot on her knee and sure enough it looks like she was stabbed with a safety pin or something similar. So I look even harder and find at least 10 more spots some old some new! I don't know what to think right now! There is a 12 year age gap in my cousin and I so we never spent a lot of time together but I started remember odd things about her. This isn't the first time my daughter has been hurt around her, looking back Im completely sickened. My mom expressed her concerns as well and told me some things that happened between my cousin and brother (8). I dont know what to think!