One night and two days of 1 night and left alone by my wife's sister who suddenly came over

My wife was pregnant and went back to her hometown, and her wife's sister came to her house, thinking she was in trouble. However, in fact, my older sister, who hadn't been going well with her husband, was frustrated and took advantage of her sister's return home to come to our house to relieve her sexual dissatisfaction. Without realizing this, I was taking a bath in a lazy manner, and suddenly came in and asked, "I'll flush your back," and he asked me, "Isn't it piling up?", but I tried to refuse a lot...