Blackmailed by my MIL

I'm 33 married with 2 children and live across the road from my Parents in Law. Last November my wife and the children went away for few days and my father-in-law (FIL) was away working at the same time. I had work to do and had a female colleague 52 come over and she stayed the night with me fucking. She was a little drunk and so was I and we ended up fucking. My mother-in-law's 56 (MIL) car was in their driveway, and I assumed she was also away. I was wrong her car was being serviced and had way for a part overnight. She saw my colleague arrive and leave and she guessed what I had been doing. Two hours later she came and asked me to take to pick up her now finished car. I didn't realize she knew about my colleague until asked me to provide her with the same attention I had given my colleague. At that time, I thought she must have seen us fucking somehow and not wanting to upset my wife by one indiscretion I agreed. After she returned home, she came over to my place and I fucked her twice. My MIL is a very horny woman and can't get enough cock from her husband and had decided to use me as a replacement when her husband was away. I thought the once would be it, but she wanted me every chance she got. FIL playing golf and my wife is out with friends she gets me to fuck her, and she had to visit her sick sister and she got me to drive her, and we fucked on the way there and the way home. Any excuse she can think of she will use; I could say I'm upset and wish it would stop. But I'm enjoying the extra sex from MIL and as long as wife doesn't know I'm happy to continue.