Is my husband dating my sister?

Dear Counsellor,

I am the first born in my family and our parents died 10 years ago in a car accident. I was left with no other option but to take responsibility for everything at home. I had to raise my four siblings, which made me drop out of school and watch them study. I got married three years ago and last year when I gave birth to a baby boy, my little sister came to help me with home chores. However, I have discovered that she going out with my husband. Last week my husband accidently left his phone home, only for me to see a message from my sister asking him where they would meet later in the day. I love my husband and I love my sister, and of all the four siblings she is the one who never gave me a hard time; instead she helped me raise the others. Please tell me what to do because I am lost for words.