She either 1. Wants to get back at your cousin or 2. She wants to have sex with you!
If your cousin's wife is playing footsie with you, then something strange is going on.
How well does she know you?
Where you around when her and your cousin were still together?
It seems that she is interested in you - whether for dating or casual sex is hard to say.
And the reason she's so interested could vary too...
If you and your cousin are close, and she knows that any sort of affair between the two of you is likely to end in your cousin finding out, then this could be part of the reason she's acting this way.
Even flirting openly with you in front of other family members or friends might get back to your cousin, so this could be her intention.
However, if she's being discreet and making an effort to hide her flirting (playing footsie is considered discreet flirting, because it's under the table and noone else can see it) - then it could simply be because she's very interested in you.
Has she shows signs of interest before? Back when she was still with your cousin?
You never know... Part of the reason her relationship with your cousin didn't work out may be down to the fact that she had feelings for you.
Now that she's single again, she's probably thinking it's the perfect time to make those feelings known - without truly considering the repercussions.
Whatever happens, be careful. When getting involved with a family member's ex, there is always an increased chance that something bad happens.