HELP! I'm Pregnant For My Fiancé's Dad And He Propose To Marry Me!

Am Folake, 28years old and I've been dating my fiancé for 2years now,we met on my 25th birthday celebration,he's a cool and nice guy,after concluding to get married,he invited me to his place,he stays with his parents since he's the only child and he's from a wealthy home.

When I got to his place,he introduced me to his dad,his dad is one of those charming old man you could ever see,he's looking so young in his 50's and own chains of businesses. I became attracted to him on the spot,he asked me couple of questions and he seems nice just like my fiancé. Our relationship was going on fine that I had to spend the weekends at his place most times,the familiarity between me and his dad grew that he now calls me his daughter,I loved it tho because it gives me that sense of belonging.

One day he came in when I was about leaving so I told him am leaving already that its been long I came,he asked me to stay a bit more and I said no that I just have to catch up on something,he went into his room and came out handed me a bundle of 1000naira note ,which is 100k,I was confused,I asked him what the money was meant for and he replied for your upkeep,I thanked him and left,2days later I got a call from him asking me to see him at his office,I went.

On seeing him he started talking complaining about women and how he feels am a nice person,I was stuned I said sir but am getting married to your son soon,he was like its not a problem,he then asked for my bank acct which I gave to him,after our discussion I left,later that day I got an alert that my acct has been credited with 500k,almost at that same moment,he called asking if I got any alert,I was so happy I thanked him and he fixed a meeting for us to see that night,this was how I started dating my fiancés dad,we kept it secret till we started having sex and I took in for him,this man actually wants me to keep the baby on the grounds that he has been looking for another child all this while,I insisted on getting rid of it and he threathened legal actions I was scared of that so I couldn't go further with getting rid of the pregnancy, he then gave me 1m to start preparing for the baby's things and for my check up too.

My fiancé don't know am pregnant reason is this his dad proposed to marry me as his second wife,which I accepted since am now carrying his baby,I think i love him he takes care of me more than my fiancé,but my fear is this how do I tell my fiancé about this whole thing that am no longer interested in marrying him.I think I prefer marrying his dad since he seem more matured and am carrying his baby. I know this will create serious tension in their family but i don't think I love him anymore like I use to,his dad don't feel they will be any problem telling him and his mum,but again am scared because I know I've betrayed him but just can't marry him.

Help me out on this,how do I go about it.