Is a person sinful if they take their niqab-wearing sisters to the market, when these sisters show their hands and sometimes their eyes become uncovered, and is he obliged to refuse to go with them?

There is no harm for a woman in wearing the niqab that shows her eyes. As for uncovering the face and hands, there is a difference of opinion. If your sisters uncover their palms, following those who permit it, then there is no harm upon them, nor upon you in accompanying them.