Is it normal to want to have sex with my uncle's wife

Well, when i was about 4 yrs (i'm 21 now ) my uncle's wife died .
After 2 or 3 years he married another woman .
The problem is with the new one.
Every time i see her, i want to sleep with her , i have fantasies about her (of course masturbation).
She is beautiful but not so hot. she's nice and all .
But sometimes she makes some very weird actions and replies (i mean socially ),our family treat her with caution in terms of what we say and do with her around (friendly but with caution ).
With all the above i don't really consider her a real family member , just my uncle's wife that i want to enjoy.
And of course i don't feel honest with my uncle at all when talking to him in any subject , and he is very nice and a good man.
They live near us and i go there a lot to give or take something.
Sometime i think maybe it's because of the porn ? but i rarely feel like this with other women around me.
And i feel so bad about it !