Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The conditions of the validity of marriage are as follows:
The woman should be fit for marriage, i.e. she should not be Mahram (unmarriageable) to the man to whom she is getting married due to the lineage, suckling, or marriage. She should not be married, or be in her waiting period after divorce from her husband or death of her husband.
If the above conditions are not met then the marriage with such a woman is invalid.
You stated that you married a woman who was already married with a man who signed the contract of marriage with her but did not consummate the marriage with her.
It is probable that this woman is married to a man and is still his wife, and it may be that he divorced her before consummating the marriage with her (before sleeping with her).
Therefore, if the first husband did not divorce her, then your marriage is void without any difference of opinion about this matter.
However, if her husband divorced her before consummating the marriage, then your marriage is correct. Because if the marriage is not consummated, then she does not have to observe any waiting period.
Allah Says (interpretation of meaning): {O you who believe! When you marry believing women, and then divorce them before you have sexual intercourse with them, no 'Iddah [divorce prescribed period, see (V.65:4)] have you to count in respect of them….}[33:49].
But something has to be taken into account. If the husband and wife had stayed in seclusion alone, then the waiting period has to be observed even if no sexual intercourse took place between them, because it is a right of Allah.
Al-Kharashi said: ‘This means that if the husband and wife were in a secluded place together, it would have been possible for them to have had intercourse, whether the fact of being alone with each other was to offer presents or just for visiting. And if he divorces her, she must observe the waiting period, as a right of Allah even if they confess that they did not have any sexual intercourse’. [136/4]
So in this case, your marriage is also invalid and she is no longer considered your wife. She is eternally forbidden for you according to the Maliki and Hambali Madhab.
However, according to the Shafi’e and Hanafi Madhab, you can marry her after the waiting period expires, and this is the preferred opinion of sheikh Al-Islam Ibn Taymiyah. So, if the man who had had his marriage contract concluded with this woman had not divorced her at the time you got married to her, then you should separate from her immediately. And according to the last opinion, you could return her after the expiry of her Iddah.
Nevertheless, if the husband divorced her as before consummating the marriage without having stayed with her in seclusion, then you do not have to divorce her. In this case you do not leave to obey your parents.
Allah knows best.