Who is more guilty?

What could I have done? What else could I have introduced in the argument? Frank knew the Sinclair family a thousand times better than me. He must know what he is doing. I provided myself with a dozen excuses to justify what I was about to do. Well, I wasn’t about to kill anyone, but the feeling was similar. 
For now, I was headed to Nathan’s room. He had only permitted me to give the servant the good news. I had left him waiting for a good while. It wasn’t the best way to start your engagement with a billionaire, but what could I do? Unwittingly, I had found trouble and a lot of filth behind closed doors.  
I could feel the sweat build up in many areas. My fingers automatically went to knock at Nathan's door, but my guilty conscience came in the way. Now, I had three secrets to hide from my fiancé. Nathan could see right through me when I had returned from Doctor Williams’ office. What would he say that time?  
The door opened a little, and a hand pulled me inside. As I wasn’t paying attention, I screamed when it happened. I was pushed against the wall, and the first thing I noticed upon returning to reality was Nathan’s sparkling hazel eyes. Under the bright light of a bulb, his irises transformed into a beautiful, prominent shade of brown, gold, and green. They looked like two unique planets.  
“Where were you?” he asked in a regular tone. 
“Uh, you..” I muttered. My inability to tell convincing lies would make my survival difficult at the Sinclair estate. “ 
“Are you hiding something from me again?” Nathan questioned. 
Shit! Why did he only notice my expressions? His mother was running around the estate like a maniac on drugs. Even a child could tell that she had committed something horrible by looking at her face.  
Would it be wrong to tell him to see her, to divert his attention from me? If Nathan learned his mother’s secret, that would completely ruin Frank’s plan. I wasn’t sure what exactly it was. All Frank had asked me to do was to meet him later so that he could formally introduce me to Mr. Sinclair. He said that he would only reveal his master plan to me after that. I hadn’t said yes to him, but I hadn’t said no as well. And I was pretty sure I was going to do whatever he said. That clever man had refreshed my fears and insecurities, and now I felt greatly troubled by my decision to trust Nathan alone for protection from the cruel world and my ruthless enemies.  
“Your silence is making me uncomfortable,” Nathan began. “I don’t want my fiancé to hide anything from me. A marriage requires trust and respect. We must tell each other everything from now on.” 
“Uh, no, it’s nothing.” I covered up my anxiety with fake laughter. “Your mother stopped me on the way to discuss some things. She asked about my parents and family. You wouldn’t know this, but my parents are long gone, and my entire family hates me. Mrs. Sinclair wants them at our engagement. I felt like my answer put her in unease. She is not happy about my situation. I don’t want to cause you and your parents shame. Does this bother you as well?”  
I was shocked at myself for so smartly changing the subject. The lies, secrets, conspiracies, and Frank's company were already impacting me in a negative yet beneficial way.  
“Why would that bother me?” Nathan raised an eyebrow. “Mother has a habit of worrying over the littlest things, and I am just glad I get to have you all to myself until you reconcile with your family.” Nathan winked at me twice and leaned in to kiss me, but romance and sex were the last thing on my mind then.  
“Your father is in the house,” I announced, hoping to make him either too upset or excited to continue.  
Nathan, however, seemed unbothered. “Great!” he remarked and kissed me anyway. He soon stopped, probably disgusted by my stiff lips and awkwardness. “Is something wrong? You don’t seem to be in the mood for this.” 
"I am surprised your father is here, and you are not excited to see him at all. I would kill to see my parents again." Now, I was manipulating him? I was on fire that day. I had to tell Frank about that. That is exactly what he wanted from me.  
“No, I am not.” Nathan’s expression intensified. I could sense some tension and anger there. “My father is not the person I expected him to be. He was spotted with a young man a few days back on a vacation we did not know about. He is cheating on my mother, and I don’t want anything to do with him.” 
Mr. Sinclair was already cheating on his wife? Wouldn’t that make things a thousand times easier for her? Internally, I clapped for her. That news would bring her much joy and help her release some tension and guilt.  
“That’s awful. Are you sure he is having an affair with that woman?” I asked with fake concern. 
Nathan shook his head. “No. A friend told me about this a few days ago. He doesn’t have any proof of a romance between them. They weren’t affectionate with each other. But what else would he be doing with a beautiful young woman at a vacation spot?” 
“I see. What do you plan on doing now?” I asked. 
“I plan on confronting him.” Nathan's hands balled into fists. “Mom has done so much for us. I can’t allow him to play with her heart. She has sacrificed her youth for him. A loyal woman like her is rare to find.” I coughed loudly at his image of her mother. He saw her as the epitome of purity. It would send him into a coma to discover her truth.  
I patted Nathan on the shoulder to comfort him. “You should gather some proof before talking directly to him. He might deny having an affair and start being more careful than before. That way, you would never have evidence of him cheating on your mother.” 
“You’re right.” Nathan touched my hand. I saw actual humanlike emotion on his face. He was genuinely worried and upset for his mother. “Don’t tell Mom about any of this.” 
“Sure,” I replied. I was lying because it was the first thing I was about to do.