פירוש על קידושין 100:11

A man cannot simultaneously marry a woman and her daughter or a woman and her sister. If he was already married to a certain woman and he attempted to betroth her daughter or sister, the betrothal would not be effective. The mishnah deals with a case where a man tried to betroth two such women simultaneously. Since they cannot both be effective neither is.

This is a classic rabbinic story, utterly packed with information. The main thing which we learn is that if one tries to simultaneously betroth two sisters, neither sister is betrothed. However, we also learn the following halakhot.
1) A man can betroth a group of women with one act of betrothal, and even if the betrothal is ineffective with some of the women (the sisters) it is effective with the others.
2) During the seventh year (the sabbatical year) a man can betroth using the agricultural produce of the women he is betrothing. This is because such produce is considered ownerless during the sabbatical year and when the man picks it up he owns it.
3) One woman can simultaneously accept kiddushin for herself and for other women.
By packing all of these details into one brief story, the story becomes an excellent didactic opportunity, far exceeding that which it is brought to explicitly demonstrate— that if one tries to simultaneously betroth two sisters, neither sister is betrothed