While changing clothes, I saw a video of sister-in-law's breasts

My sister-in-law was changing clothes in her room. I was sneaking at her from the corner of the window. Sister-in-law's back was on my side. She was wearing her top. With a style she brought her hair towards her back. Most of her back was still lying naked. Sister-in-law got a little slanted so I saw her swinging breasts. Just looking at her, my pants became tented. It seemed like going and holding sister-in-law from behind and doing a lot of kissing on her bare back, placing lips on her back.