Is it true that widows who don't practice celibacy after death of husbands go to hell ?
Not necessarily. A widow has three options after the death of her husband. Either she can follow celibacy (Vidhwavrata), can marry an other person or perform sati.
Naradha Smriti 12.97 “When her husband is lost or dead, when he has become a religious ascetic, when he is impotent, when he has been expelled from caste, in these cases a woman may be Justified in another husband.
Garuda Purana 1.107.28 “In case of disappearance or death or renunciation or impotent or lost caste status of her husband, in these five cases a woman is allowed to take another husband.”
Agni Purana 154.4-7 “Women are allowed to have another husband in the following five adversaries;- (the first husband) is lost, dead, has become an ascetic, impotent or fallen morally. If the husband is dead, she should be given to the brother of the deceased. In the absence of brother, she should be given to anyone as one wished”