Szwagier cieszył się majtkami szwagierki, widząc okazję

The brother-in-law liked his sister-in-law but was afraid to tell this to the sister-in-law, whenever he got a chance, he used to enjoy the sister-in-law's undergarment, then one day the sister-in-law caught him red-handed and then the sister-in-law let the brother-in-law do everything he wanted to do