Suddenly the woman turns to her (“their”) sister, who is young. She describes her sister’s body and then compares herself to her sister (vv. 8-10). He does the same — compares himself to another — in vv. 11-12. Here are the first set of words:
8:8 We have a little sister,
and she has no breasts.
What shall we do for our sister,
on the day when she is spoken for?
9 If she is a wall,
we will build upon her a battlement of silver;

but if she is a door,
we will enclose her with boards of cedar.
10 I was a wall,
and my breasts were like towers;
then I was in his eyes
as one who brings peace.
V. 10 makes it clear that the woman is the speaker; this suggests that vv. 8-9 are also her words.
Unlike the little sister, this woman needs no preparations. The girl’s chastity seems to be the issue — though we cannot be sure. It seems to me that they are speaking of protecting the young girl from invaders.
The woman knows what it was like to be chaste and to be protected by society; when she was young and protected, she was (now to re-quote her words): “then I was in his eyes as one who brings peace.” She “opened her door” and this brought in her lover in peace — there was no longer need for protection. Those days were over.
Now this theme of sexual availability that is being offered to one and only one other is central to the book. Here she says that she has given herself to her lover, and to him alone. (Solomon can sulk should he wish to have her.)
Even if the “little sister” is fictitious, which some think, the woman uses the little sister as a foil for her own self-perception in front of her lover.