Am I obligated to marry a young man with whom I committed major sins and transgressions, and who caused me to disobey my parents and sever ties with my sister, simply because I love him, or should I separate from him to please my parents and gain my Hereafter, especially since separating from him might deprive me of the opportunity to marry in the future?

Islam came to preserve the dignity and honor of women, and thus warned against establishing relationships between the sexes outside the bounds of marriage. You erred by getting to know a young man online, which led you into disobedience. You must immediately sever your relationship with this young man, for he is not suitable for marriage, has obtained what he desired from you, and will seek another to marry. Your relationship with him has caused you to disobey your parents and have a poor relationship with your family, which are greater sins. Your only path to repentance is to sever ties with this young man, return to your parents and seek their forgiveness, and change your place of residence. Do not fear the future, for whoever fears Allah, He will make for him a way out and provide for him from where he does not expect. The pleasure of Allah and obedience to parents are worth sacrificing any fleeting desire for.