Surprised by the mother in law! ....Part 1

This is 100% true and it happened to me on sunday. To keep it fairly brief my wife and her mother had been invited to a bbq in the town we live. My MIL lives 15 miles away...it was an occasion for them both to drink etc and my MIL was to stay here on sunday night. Now..my MIL had always been pleasant enough but i always felt she didnt approve of me..like everybody elses MIL i guess!...Shes very well spoken, an average looking women for her age...shes 65 although she looks younger. Im 30..my wifes 39. Anyway...trying to keep it as brief as i can my MIL arrived etc and off they went to the bbq which was a 2minute walk away. Great...football..beer etc i was sorted. OK..the first piece of this story came when i went to have a shower. I know its wrong but i saw my MIL bag in the spare room and i just HAD to have a look you know?!..usual items...washbag...change of clothes and of course underwear..bra and knickers. I was intrigued..i just had to see the size of her bra etc which i did and looked at her knickers..bog standard white boring knickers and bra set....no surprise there. But the fact i was touching my MIL underwear really turned me on and so i went to the bedroom with her knickers and had a wank as im sure everyone would. Anyway i got showered and was upstairs when what i thought was my wife came upstairs...it was my MIL...panic...!