1. No, it is not true. Wife is not legally considered as the joint owner of the property and can not claim ownership of the said property during the life time of the husband.
2.However, the wife inherits share of her husband's properties along with other legal heirs, in case her husband dies intestate.
3. For claiming the title of the property, the said woman shall have to apply for and obtain grant of probate from the Court with out which the value of the will be not more than a scrap paper.
4. If she file an application seeking grant of probate, all the legal heirs of your deceased father will get notice from the Court and in that case you shall be able to challenge the will fittingly on appropriate grounds.
5. In case of properties located at some places like Delhi, no grant of probate is required and in that case you shall have to challenge the registered Will after picking up loopholes in the said file.
6. Your father could have been accused of committing bigamy but his will can not be considered as void for his said bigamous activity.