My niece slept with my husband. What do I do?

I’m so confused right now I honestly don’t know what to do. My husband and I have been living with my teenage niece. Her mother, who is my sister, lives in the village and had asked if we could live with her so she could go to a better school. I was relieved when my husband agreed to it because it was going to lift a huge burden off my sister’s shoulders.

I recently noticed she’s been very rude to me, often picking a fight with me. Whenever I ask her to do something she either drags her feet about it or flatly refuses. I decided to send her away to her mum since it was the holidays so she can cool off or something.

When I went to visit my sister the girl was still acting strange and at some point she mumbled something about my husband finding her better. On digging deeper, she said she had slept with my husband several times and was in love with him! This girl is 16 for goodness sake!

I’m shattered, angry, confused, and sad. How could my husband do this to me? Am I not enough? What should I tell my sister? Should I report my husband for sleeping with a minor? How do I come back from this? Help!