Halakhah for Yevamot 108:8
אלא העראה דכתיבא גבי אחות אב ואחות אם למה לי
that is derived from a positive one<span class="x" onmousemove="('comment',' Her husband's brother shall go in unto her after the death of his brother. ');"><sup>19</sup></span> it has only the force of a positive commandment!<span class="x" onmousemove="('comment',' The penalty for the transgression of which is not that of kareth. Whence therefore can he traced in the Bible that the penalty involved is kareth? ');"><sup>20</sup></span> — Scripture stated: <i>And if a man shall take his brother's wife, she is a menstruant</i>.<span class="x" onmousemove="('comment',' V. supra p. 359, n. 5. ');"><sup>21</sup></span>
Sefer HaChinukh
From the laws of the commandment is, for example, that which they, may their memory be blessed, said (Yevamot 54b) that the prohibition of the sister of the father is whether she is the sister of his father from the father only or from the mother only; whether she is the sister from marriage or whether from licentiousness - as each one of these is called his sister regardless. And there is no room for secondary [prohibitions] here.
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