Am I obliged to marry my paternal uncle's daughter whom I touched, knowing that I am unmarried and will soon marry another girl, and can she legally force me to marry her?

You must seek Allah's forgiveness (istighfar), repent, and resolve not to return to the sin. Touching a woman does not necessitate marrying her, as a marriage (nikah) is only valid with a contract that fulfills its conditions, including the consent of the husband. There is no need to inform the parents or the girl. Rather, what is obligatory is to conceal your sin and repent between yourself and your Lord, according to the Prophet, peace and blessings be upon him, who said: "Avoid these foul deeds which Allah Almighty has forbidden. So, whoever commits one, let him conceal himself with Allah's concealment."