How can a betrothed woman maintain a lawful relationship with her sick fiancé that does not displease Allah, given her knowledge of the difficulty of completing the marriage at the present time?

You have no control over your own affairs. The actions and choices of a servant in their life are governed by two fundamental principles: belief in divine decree, both good and evil, and that the servant is commanded by their Lord.

The suitor's illness may be a sign that marriage to him is not suitable for you, and perhaps this is the fruit of your Istikhara (prayer for guidance).

A marriage project that is "on hold" indefinitely is difficult to accept Islamically, as it does not fulfill the objectives of marriage, such as procreation and chastity. Furthermore, concealing the illness from your family is considered a defect, for the guardian (wali) is legislated to express an opinion that serves the woman's best interest.

You must inform your family of all the details of this serious matter, as they are your partners. A woman may be swayed by her emotions, so the voice of reason is necessary to guide her. If the guardian refuses, Islamic law permits him to prevent it.

The suitor has no right to prevent you from informing your family, nor does he have the right to inflict his affliction upon people; for it is among the noble morals that a person should prioritize others over themselves.

If the guardian agrees and you reflect wisely, then your discernment takes precedence. Know that you are strangers to each other until the marriage is concluded, so it is not permissible to be overly expansive in speech and interactions.

No one can force you not to await relief from Allah, but observe Allah in your relationship with Him, think wisely, and do not act unilaterally without your family and legal guardian.

If it becomes clear that it is better for both of you to end the relationship, your guardian should handle the matter wisely to minimize harm and damage. Whatever your choice, seek help from Allah.