Picking up the shirt of sister-in-law who was cooking, I fucked her from behind

My sister-in-law was cooking in the kitchen. For 2 days brother was not home either. I went to sister-in-law and started helping her in cooking. Then something happened in conversation that I lifted sister-in-law's shirt up and slipped her salwar down. Sister-in-law was not wearing tights inside. I was desperate for a long time to see her circular buttocks naked.