The stepdaughter is not a mahram for the son of her mother’s husband. So it is permissible for you to marry your step sister and there is no problem in that

While Islam encourages timely marriage to safeguard chastity and uphold moral values, there may be circumstances that lead to a delay. However, such delays should not be prolonged unnecessarily, and communication with family is essential.

A woman can’t have more than one husband in Islam simultaneously because Allah has made a woman like a vessel, but man is not like that. If a woman becomes pregnant when she has had intercourse with a number of men at one time, the father can never be known. People’s lines of descent and lineage will be mixed up, families will be destroyed and children will be lost. Women will be burdened with so many children that they will be unable to bring them up and spend on their maintenance. Maybe women would find themselves forced to sterilize themselves, which would lead to the extinction of the human race.

Marriage is obligatory for a woman who is unable to feed or clothe herself unless she gets married or she fears falling into immoral acts if she does not marry. What a woman can do to fulfill this obligation is not to refuse marriage when a suitable, compatible man comes with an offer of marriage.