I don't get some people

I'm co parenting at the moment and a few weekends ago him and his mum had her on Sunday for 5 hour's and when we picked her up they told me about a lump and bruise on her head.
His mum turned to my 6 year old and said to her have you battered your baby sister to her face. I will say it wasn't there in the morning.
How can someone be so horrible to blame a kid for what they have done and they didn't own up to it. This is the 4th issue I have had with them