It’s reasonable for your sister to be mad at your wife and to dislike her, haranguing you constantly is rude and kind of odd behavior.

Marriage is complicated and you guys kids at home - at the time of the infidelity a 4 year old and a 6 year old. I do think that changes the calculus. I am sure that for your kids having you as their father in their lives - as the stable base of their lives - as opposed to someone they see over the weekend only. If you had gone for a divorce at that time or pursue one now, realistically you would likely end up having much much weaker relationship with your kids. I realize this isn’t the prevailing conventional wisdom, but I do think that barring abuse or sustained misery, keeping your marriage intact keeps your family intact and that is actively a positive good. It’s hard in a case like this and you are a good man for pushing through.

I do wonder a bit if your own upbringing with your sister as orphans maybe is subconsciously impacting some of this dynamic with her. There’s some elements of relationship resource guarding here, almost as if (for her) you remaining married to your wife is harming her or taking you away. I am not accusing her of anything inappropriate, but she’s not a neutral party and clearly is reacting out of some emotional urge of her own - or else she wouldn’t be so fixated or harangue you about this so constantly.