Premarital relationship

I am in a relationship with a girl who is my relative since the past 7 months and I really love her a lot and have strong plans of getting married to her. We got physical a few times as in kissing and touching but never in intercourse. I even talked to my parents about this but my father doesn't agree due to relationships going wrong between her families. My question is how do I convince my parents on this and will I be considered as disobedient if I go against them and is there any way I can repent for my sins. I am really confused and I want to get married to her in accordance to Islam and can't think of any alternative.