Though English is my second language, I feel like there is a riddle of interchangable grammer in your senteces. If I were you, gramatically I would write in this way :
Not only I am ( not am I) in love with a married man, the worst (not worse) is that he's my uncle.
Is there any (not a ) better way to express this idea.
And, I want to know how (not what) someone (not one) calls the person who is having an affair with someone's husband? Stealer? Usurper?
Those changes are accordingly to my understanding, however I will appreciate anybody`s opinion to furnish my perception.