Who is more to blame, the homewrecker or the person who is supposed to be in a committed relationship?

My brother-in-law was married once, divorced, and then he got married to my sister. Their marriage was going great, had two kids, and was a happy family. My brother-in-law’s ex blamed my sister for wrecking her marriage because his ex claimed that my brother-in-law was cheating on her with my sister while they were still married. Now, his ex found a way back into my brother-in-law and sister life. Apparently, my brother-in-law is still fucking his ex while married to my sister.

Who is more to blame, the homewrecker or the person who is supposed to be in a committed relationship?
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