Would the estrangement of a sister from her in-laws be an impediment to the marriage of her daughters, despite their commitment and good character?

The wife's patience with the harm caused by her husband's family, her overlooking their faults, and her honoring them are considered part of a good marital relationship. Estrangement is dispelled by greeting them, calling them by phone, and visiting them on special occasions. There is no obligation to visit them weekly or monthly. Therefore, a woman is not considered to be severing her husband's ties of kinship if she connects with them on occasions and otherwise.

As for the daughters not marrying, it might be better for them, in accordance with the Almighty's saying: "And it may be that you dislike a thing which is good for you and that you like a thing which is bad for you. Allah knows but you do not know." It is not necessary that the delay in marriage is due to estrangement, even though the ill effects of disobedience might extend to others, and a person might be deprived of provision because of a sin.