The Fight and Deal that followed

I'm Neil 35 married 2+ children, my wife Kristen 33 is in the the last month of her pregnancy with our third child. Early last month she found out I was fucking a woman at my work, which caused a fight. But it wasn't a sexual relationship with the other woman Joanne 53 divorced, it was just one fuck. Joanne was horny and I was willing and Kristen found out by me not being careful and forgetting there was lipstick on my underwear. Joanne was already leaving to live in another state and at the farewell party at the office we hooked up and continued it later in my office after work. Kristen was upset that I had fucked outside of our marriage and not not kept it inside the family. At first I thought she meant herself, but she didn't. She meant Susan 51 her widowed mother, she said I could've said I was feeling horny and she would've gotten Susan to service me (her words). I was confused by her response at first, Kristen is the oldest of 2 and Susan lives with us in annex. Susan does the cooking, washing and house work to Kristen during her pregnancy. Susan is better looking than Joanne and I hadn't thought of her as a sexual partner. At first I thought Kristen was having me on and when Susan came in. Kristen told Susan I was feeling randy and needed servicing, I waited for for Susan to say what's that got to do with me or something similar. Susan said I'm busy with dinner now, but will join the 2 of you in your bed later on. Susan left and I asked why would she do that, Kristen said because she's horny and also she loves me. That night I fucked Susan in bed besides Kristen and she has shared our bed ever since. The only catch if you can call it a catch, is that I'm to regularly service Susan from now on until she doesn't want me too. I'll know because she will move back to her own bed I've been told by Kristen. I asked will we be having threesomes and all I got was a nod of approval. Best wife and mother in law ever.