To desire a sexual relationship with mother in law (wife's mom)

Been dating my (now wife) since high school. We got married in our 20's and we are now both in our 30's. After a couple years of dating, I started to notice her mom. I was a teenager at the time. She was in her mid 40's. Her mom is now almost 60. She's single and lives alone. Hasn't been with a man for about 10 years.

The years have been kind to her mom. Everyone thinks she has still 'got it' for her age. And she does. Big Round Butt. Soft smooth skin. Beautifully shaped thighs and adorable hands. Awesome boobs too. On top of that, she has a super sexy accent and is very goofy. She is self conscious about her midsection cause she's gotten a little flabby there, but I think she minds much more than I do.

We are all very close. The 3 of us hang out all the time and have for years. She really likes me as a son in law. She's always been sorta flirtatious with me, but it was never ever spoken of. Little glances. Smacks on the butt (mutual). One time she sat on my lap while I was at the computer. Her daughter (my wife) walked out of the bathroom and saw that and didn't think nothin of it.

A few years back, she had been experiencing some discomfort in her lower back right above her butt. (for real - she has some sciatica pain). Then one day (still a few years ago), the most awesome thing happened. I had gone with her to pick up her plasma tv and went back to her house to help her set it up. We were alone. She complained of some pain and I told her to lay down on the floor so I could rub it out for her. She laughed, but went on her stomach. I asked her where the pain was, and she seriously pointed right on her butt. So I started to massage it with my right hand and actually put my left hand right in the middle of her buttcrack (pants on) for support. After about 1 minute, she said thanks and sorta got up but put her butt real far back(like doggystyle position) and made it very clear that she was comfortable and even happy to have me behind her with her head down and everything. I was right behind her just amazed out how cool this was. I continued rubbing her butt and had one hand between her legs and sorta kinda rubbed her pussy through the pants but really subtly.

She then stood up and I smacked her butt and said (exact quote here).. "dang mama, nice booty!". She laughed and walked away and that was the end of it.

Fast forward a bit. Nothing more ever happened after that. And we have never spoken about it. But when I give her a hug, I always sorta bring my hands lower and feel her booty a little. She knows it, and has never minded. But the last couple times, she kinda blocked my hand! ah!

To cut to the chase. Here is what I gather (with observations that reach far beyond what I can fit into this descriptive question). She is scared because she thinks I am gonna ruin my relationship with her daughter, which is not my intention. Since lately my MIL has been sorta weird and standoffish to me, should I just give it up and forget it? Or should I let her know that I think me being attracted to her actually makes me more attracted to her daughter and that it's really a good thing for everyone (that's true btw). My wife is hot.

In a conversation a while back, my wife asked me if I thought her mom was pretty. I said yes and then I said that I love her mom's butt and have fantasized about her. Basically her response was "Well... is this going to have a negative impact on our relationship?" I told her no it won't. Then she even said that's a good thing because that means I will be attracted to her for a long time. But she doesn't know that I fantasize about her mom ALL the time. And I think she would not be happy if we 'did' anything and wife found out. The truth is I want to do every naughty thing you can imagine with her mom and I cannot stop wishing for it, though I've tried to over the years.

I gave some advice to a person on is it normal from another question to communicate open and honestly, but man it's hard when it's your own scenario and you don't want to screw everything up.

Here's what I wish would happen. I tell MIL that I have the hots for her and that I want a sexual but non-romantic relationship with her, but that it's a good thing because that means I will have the hots for her daughter for a really long time. After all, my wife really did say that, so I wouldn't be lying. Maybe that might make her more comfortable with us foolin around all through her 60's.

What should I do?