I need counseling

5-10 years is where the marriage magical spell tries to loose its charm..First Priority is get your wife out of depression, take one or two weeks leave and spend quality time with her..Go to places even international is also fine like Georgia, Vietnam depends on budget.. Listening is very important, understand her perspective, check what makes her think like that.. fake it till you make it..

You should take help of marriage counselors who genuinely try to save marriages

Harmones imbalance. Try to go on vacation, ask your wife to join some fitness clubs or yoga , try to organize small small events to make her happy.

There is a lot of context missing in your post. Why does your wife feel you have a special feeling towards your SIL. Have you tried finding that out? When she communicates her feelings, have you tried putting aside your feelings and just listen to her vent. When she is venting, that's not the time for you to talk about how it's making you feel or justify your actions. In what way do you feel you had a happy marriage until now and it has changed suddenly. Guys are generally poor in reading the cues in relationship. Mostly because they are more concerned about themselves than their partners, and when things boil over and take a bad turn, you feel it came out of nowhere. Did you ever pay attention to what was ailing your wife all through the past 5 years? She didn't get into depression overnight. Disappointments were building inside her since years and she doesn't see a way out of it. In what ways have you shown up for her, that'll give her the confidence that you are with her not against her. If her confidence in you is broken, it'll take time and right actions to heal.

Y is ur wife getting that feeling that u have special feeling for brothers wife?

Avoid whoever your wife feels uncomfortable why can't you guys just do that if marriage and relationships matter so much why can't you sacrifice a little...start practicing you can save your marriage and your wife will feel better she is under pressure postpartum is very depressing... understand this

Assuming you really have no connection to your SIL.

I am not an expert but here is my thought.
Every person wants to be cherished, they want to feel important, they want to feel secure.

Most men are working, so they implicitly become important, their job , their status, their schedule becomes a priority around which life of others revolve.

In such cases I have seen ladies feeling neglected, they become insecure as they feel they are not equal in relationship and no one is dependent on them.

So, if she has some hobby, some craft, some education that can be utilised to get her some work/business it will relieve her of many insecurities. Take active participation in something that your wife likes.

Eg: My wife likes skin care products, I never had more than a soap & shampoo... I reasearched the products, I understood them.. now I know more than her, we do skin care together...

Also, eat well, exercise together, ample of sun - not kidding.. this can trigger some good hormones in people..

I have experienced it first hand with my wife.

2 years of kid time would have had enough stress on her. In this time sof nuclear family setup kids add a lot of stress. It's a mixed feeling and no me time that creates all problem. As many suggested vacation and dedicated time with family should help. Plan weekly family time as well and be very committed to planned time. Her confidence should boost with your efforts and things should sort out. All the best.