The answer is not while her husband was living.
Married women did not own property, it was quite simple, everything went to the husband and he managed it. If their heiress's parents were careful, they set up a transfer of the property to both husband and wife, their daughter, so that while the husband managed everything, he could not sell.
This way the inheritance could not be squandered.
There are even occasions when an heiress wife had to ask her husband permission to leave gifts in her will to her children.
During their life together, if the husband had to go away, to battle perhaps, he might well leave his affairs in the charge of his wife. Then she was indeed mistress of her own affairs. But if the husband died first, not quite so common as today due to the greater occurence then of death in childbirth, the widow then could own all the property she had brought to the marriage and even all the husband's property as well, if he had left it to her. The exception to this was that the husband's land, his real estate, would go to his heir, regardless.
So the widow could be a wealthy woman managing her own affairs. Some of the wealthiest, tenants in chief of the king, even paid the king to not give them in marriage to another husband. Other widows went into convents, possibly because they found them convenient old-folks homes.