I slept with my husband's brother?

My husband has been out of town for work, for the past 3 nights. Last night I was home drinking by myself. I heard my doorbell and when I answered it was my husband's brother. He came in and I was already drunk, I asked if he wanted some wine drink. He said yes. And when I was in the kitchen pouring the glass he came in behind me. And pulled off my husband's boxers. And we had sex!!!! On my counter!!!!! And my husband comes home in 2 days and I feel so horrible!! Should I tell him? What should I do please don't judge it was an honest mistake and I would never do it again