Yevamos chapter 14, mishnah 3. In the previous mishnah we learned that when two brothers are married to two sisters and one of the brothers dies, his widow is exempt from yibum and from chalitzah, because she is the sister of the yavam's wife and is therefore forbidden to him as an ervah. Our mishnah comes to teach that this law applies even where the marriage is valid only by rabbinic law.
Two deaf-mute brothers married to two sisters:
The mishnah lists three possible combinations:
To two deaf-mute sisters.
To two sisters of sound senses.
To two sisters, one a deaf-mute and one of sound senses.
In all these combinations the status of the women makes no difference: since the brothers themselves are deaf-mutes, both marriages are valid only by rabbinic law.
Two deaf-mute sisters married to two brothers:
A further group of cases, and here too there are three combinations:
To two brothers of sound senses, fully capable in every respect.
To two deaf-mute brothers (deaf and unable to speak).
To two brothers, one a deaf-mute and one of sound senses.
"Harei eilu peturos min hachalitzah umin hayibum" - these women are exempt from chalitzah and from yibum.
In each of these cases the marriages of both couples are valid only by rabbinic law, and therefore the widow is exempt from chalitzah and from yibum because she is his wife's sister, even though she is his wife's sister only by rabbinic law. For since the entire marriage is rabbinic, the obligation of yibum is likewise only rabbinic.
And if they were unrelated women:
If they were not sisters to one another but women with no family relationship between them, "yikansu, ve'im ratzu lehotzi yotzi'u" - they may enter into yibum, and if the yavam wishes to divorce them after the yibum he gives them a get, and the get is valid. This is so even though the entire marriage rests on the force of the zikah from the brother, which itself is only rabbinic; that presents no difficulty, since everything here, both before and after, is rabbinic alone.
Chalitzah, of course, cannot be performed in these cases, since a deaf-mute man and a deaf-mute woman are not fit for chalitzah.
In summary: in this mishnah we learned that the exemption of a wife's sister from chalitzah and from yibum applies even when the marriage is valid only by rabbinic law - whether in the case of two deaf-mute brothers married to two sisters, or two deaf-mute sisters married to two brothers, in all their combinations. And when the women are not related to one another, they enter into yibum, and if he wishes to divorce them he gives a valid get, except that chalitzah is not possible with a deaf-mute man or a deaf-mute woman.