what seems like genuine love is just lonliness really. some things ive picked up in your message:
-you are lonely because your family is not around anymore. you mentioned you had no one to talk to
-your wife laughed this off as a passing cr\ush when really you have what appears to you as genuine feelings. perhaps her laughing this off is symbol;ic to something much deeper-your lack of connection with your wife and to each other's feelings lately/.
-last but not least you feel like you have always been close to your sister in law and thats not reality ,
real relationships require sacrifice and real life drama,. this relationship with your sister in law is one of an illusion. you only see her on the good days. has she gone through what you and your wife have gone through these past 10 years-? maybe children? the death of your loved ones? the constant support and standing by ? that is real love. not companionship like your sister in law shows you, that is friendship. value what you have and seek to make this bond stringer with your wife by going to counseling together, sharing deep feelings, listening, and taking time for the little things.
you must also realize this secondary relationship is a facade and you must distance yourself from this woman.