Talmud for Yevamot 214:2
איתיביה רבא לעולא כל שיכולה למאן ולא מיאנה צרתה חולצת ולא מתייבמת ואמאי תמאן השתא ותעקרינהו לנישואי קמאי ותתייבם צרתה צרת ערוה שאני דתני רמי בר יחזקאל מיאנה בבעל מותרת לאביו ביבם אסורה לאביו
who did not exercise her right, must perform the ceremony of <i>halizah</i><span class="x" onmousemove="('comment',' With the levir, though he is the father or any other forbidden relative of the minor. It is only the rival of a woman whose marriage is Pentateuchally valid who is exempt from both levirate marriage and halizah with the forbidden relative of that woman. The marriage of a minor, who could exercise her right of refusal at any moment, is only Rabbinically valid. ');"><sup>4</sup></span> [if her husband died childless] but may not contract levirate marriage.<span class="x" onmousemove="('comment',' Supra 2b. Since after all the minor did not exercise her right of refusal her marriage is valid enough to forbid her rival's levirate marriage, as is the case with a Pentateuchally valid marriage. ');"><sup>5</sup></span> But why? Let her exercise her right of refusal now and thereby annul the marriage of her first husband, and then let her rival<span class="x" onmousemove="('comment',' Who, by the declaration of refusal of the minor, ceases to be her rival. ');"><sup>6</sup></span> contract the levirate marriage!<span class="x" onmousemove="('comment',' With the minor's forbidden relative. ');"><sup>7</sup></span>