I Cannot Get My Head Around This.

Fucked my fiancées mother four months ago when she found out somehow I'd wrongly had very drunken sex with an ex of mine at a party. Well that's what I was told anyway, as I don't remember any of it at all.
I thought my future mother in law was going to tell her daughters after shouting at me, but instead she hitched up her skirt, removed her knickers and told me I'd better make her orgasm.
Over the next hour I licked and tongued her pussy, clit as well as her arsehole and fucked her in lots of difficult positions, this after she'd spent an age sucking on my cock and balls.
By the time I'd cum inside her for a second time I was utterly knackered.

A few days later still fearing she'd end my relationship with her daughter, she called by our flat. Once again we had sex with my fiancée still having three hours left of her shift.
Again I was exhausted, but the sex had been absolutely amazing. But my fiancées mother stayed until her daughter got home.
They had a long conversation in the kitchen as I sat in the lounge and when they walked in, my fiancée said "We (meaning her and her mother) know you didn't sleep with K***. But now you've enjoyed mum, I hope any chance of you possibly cheating has gone".

Now I'm not sure even now where all this came from. I'd not once thought about cheating on my fiancée. But what I do know, is they'd set it all up to have me fucking my future mother in law.
I still can't figure out how it benefits my fiancée having me fuck her mum, but four months on and my fiancées mother visits twice a week for sex.
I'm not complaining, but I genuinely don't understand women. Confused doesn't even come close.