I knew that my sister's marriage was not going that well. I also knew that my brother-in-law was a great guy so wondered what was wrong. Don't ask me how but my mother found out that sister was not that outgoing sexually so my guess was that BIL was not getting satisfied. One day when I knew that BIL was not at work I called and asked him to drop by at my house on the pretense of asking him to check my car out. He's a good amateur mechanic and actually there was nothing wrong with the car. He checked around the engine getting his hands oily (it's an old car) but of course found nothing. He went in to wash up and I suggested that he try out the new shower. At first he said that a hand wash would do but I persuaded him. From that point on it was easy. Once he was in the shower I stripped and, without a word, got in with him. He soon got hard, especially with me rubbing against him. I couldn't resist so I dropped down to give him a blow job and soon he popped his cum in my mouth. Afterwards in the bedroom we simply fucked for the rest of the afternoon. He said that my sister would never do the same for him. Now, whenever I need something 'fixed' he comes over and we 'fix' each other. Sister and brother are now getting along much better. Does my sister suspect anything? I don't know but she doesn't say anything.