Yevamot 29A

והא אמר רבי שמעון: שתי אחיות לא חולצות ולא מתייבמות!

This halakha was also taught in the previous Mishnah (26a), and it is cited in this Mishnah according to the opinion of R' Shimon. The Talmud asks: But didn’t R' Shimon say: Two sisters neither perform ḥalitza nor enter into levirate marriage? Because these women are eligible for levirate marriage according to Torah law, they are considered the sisters of a woman with whom he has a levirate bond. In this case, R' Shimon exempts them from both ḥalitza and levirate marriage. Why, then, does the Mishnah require them to perform ḥalitza?

גזירה משום איסור מצוה דעלמא.

The Talmud answers: It is a rabbinic decree due to the general case of a prohibition resulting from a mitzva. If they did not perform ḥalitza in this case, where they are sisters, there is a concern that people would assume that their exemption from ḥalitza stemmed not from their status as sisters but rather due to the prohibition. This would lead to the mistaken conclusion that women who are forbidden due to a mitzva do not require ḥalitza. Therefore, the rabbis issued a rabbinic decree requiring ḥalitza in this case.

הא תינח איהי, אחותה מאי איכא למימר? גזירה אחותה משום לתא דידה.

The Talmud asks: This works out well for her, as she is forbidden due to the prohibition resulting from a mitzva. It is appropriate to require ḥalitza with her, to demonstrate that ḥalitza is generally applied in cases where there is a prohibition resulting from a mitzva. But with regard to her sister, what is there to say? Why must she perform ḥalitza as well? The Talmud answers: There is a rabbinic decree with regard to her sister due to her.