Help please, I need advise!!!!!

Today my husband came home from work. He works out of town, I asked him if we could mow our yard because we are suppose to be getting rain. I can't do it my self because I am 37 weeks pregnant. And we have several acres. Long story sorry! Well as we were done he said let's go to my sisters we go her yard looks like a jungle. Our 5 yr old daughter is with us. He said let's go home get our riding mower and help her. I agreed, she lives down the road. So as we are mowing her yard, my daughter is inside the house with her kids her 12, 6, and 2 yr olds. As we get done not even 30 mins later I go inside to get my daughter. My daughter is crying I asked her what's wrong she doesn't respond, I ask the 12 yr old and he said she couldn't get of the top bunk bed and I was helping her. I ask my daughter she goes outside and crying. My husband ask me what's wrong with I said idk. Again the 12 yr old said she could get off the bed. We head home I sit there and ask her I know you what happened baby. Mommy can't help u if u don't tell me, my husband asked her again. She said is will tell u mommy. He wouldn't let me leave and told me to pull my pants down. I am like did he touch you any where she said no but he wouldn't let me leave. So I tell my husband, he runs back to his sisters house and talk to them. He comes back and tells me he confessed and said did exactly what my daughter told us. I am heart broken because it hurt to see my daughter cry. It hurts because I feel like I let her down and she had to go through that. I feel horrible. Husband tells me not to judge his nephew because there is obviously something wrong with him. But this is my daughter. Help what can i do??? Sorry to long!