Is it a sin to take a female cousin who is wearing body-revealing clothes to the market, knowing that she will not buy Islamic compliant clothing? And does the sin cease if accompanying her allows for an attempt to advise her to lessen the objectionable behavior, and if she is not convinced, is it obligatory to refrain from accompanying her?

May Allah reward you for your zeal in upholding the boundaries set by Allah and your eagerness to enjoin good and forbid evil. Accompanying your relative to the market to buy un-Islamic clothing is not permissible, because it constitutes cooperation in sin. Embarrassment or fear of her anger is not considered a justification. However, if it is for a permissible purpose, there is no harm, provided you are keen to invite her [to righteousness] and explain the seriousness of tabarruj (immodest display). If your presence will mitigate the evil and may be a reason for her to commit to the shar’i hijab, then we hope there is no sin in that. You must strive to advise her with wisdom and gentleness.