A lot of other posts seem to be focussing on advising you in your situation, rather than on answering this (admittedly loaded) question. Then I’ll bring it back around to ‘advice,’ such as it is.
Q:
Is it a sin for two people already married [to other people] to be in love with each other?
A: It all hinges on the meaning of ‘in love’. It is a colloquialism that has a range of meanings, so to keep it short, I’m going to assume it means being fully emotionally invested in the other, but (in the context of the question) without having allowed any physical infidelity that would be obviously adulterous and sinful.
What is left has two components, stemming from the emotion and the will. The emotional aspect is not sinful. I won’t go into detail.
The willful aspect
is sinful in a twofold manner, and dangerously so. First, since both persons are married, there is a direct violation of the ninth commandment.
Second, it would seem from the common idea of being ‘in love’ that this movement of sensuality must be being meditated upon to some extent, whether through a deliberate (positive) action or a (negative) failure to reject a sinful passion. The technical term is
morose delectation. From the same link: “deliberately holding and turning over what should have been cast aside as soon as it touched the mind.”
A quick and dirty translation to your situation as I understand it: it would appear that you have every reason to pursue separation, divorce and annulment from your husband. That is an utterly separate issue from the involvement of a third party who is also already married. By all means, get out of your abusive marriage. But cease and desist this other relationship.
If I was Dr. Phil: “Don’t do it for him, honey. Do it for you.”