All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First of all, all perfect praise be to Allaah, Who enabled this girl to repent from masturbating and we ask Allaah to reward her and grant her a good husband and bless her with righteous children who would be a delight to her eyes. We have already explained the ruling of masturbation in Fatwa 81150, so please refer to it.
Therefore, we advise this girl to take it easy as the matter is very simple and does not necessitate this worry and stress. She should not be afraid to the extent you described as this may have bad consequences on her.
What she used to do does not mean that she necessarily had broken her hymen [and we do not think that her hymen is broken]. Even if we presume that her hymen is broken, she is not obliged to inform the person who proposes to marry her about it. If we presume that he asks her, she may use Tawriyah (saying something which has more than one meaning and intending a meaning different from what the listener is likely to understand) and she should not be frank about it. The hymen could be broken due to many reasons, like a sudden jump, menses and the like. For more benefit, please refer to Fatwa 86394.
Furthermore, it is not permissible for her to tell her husband or anyone else about the sin of masturbation, as it is an obligation on the sinner to conceal his/her sin. The Prophet said: "All my nation will be forgiven except those who openly expose their sins. Exposing one's sins is for a person to commit a sin during the night while Allaah had concealed it from the public, yet, he comes in the morning, and says, 'O so-and-so, I did such-and-such (evil) deed yesterday,' though he spent his night screened by his Lord (none knowing about his sin) and in the morning he removes Allaah's screen from himself." [Al-Bukhari]
Therefore, this girl should seek the help of Allaah and ask him earnestly to facilitate marriage for her and better her marital relations with her husband.
Finally, we advise the questioner to help her sister in Islam and remind her about anything which will help her to get rid of these whispers, like seeking Allaah's Refuge from the devil, strengthening one's faith in one's heart and the like.
Allaah Knows best.