1) The Gemara tries to prove the culpability of Haaraah from Vayikra 20,18 dealing with Nidda, which uses the term haaraah and from one's aunt - Vayikra 20,19 which uses the same term. Should the gemara not answer Shnei Ketuvim Habaim KeEhad?
2) "Vei Katav Rahmana beAChos Em she'ken Vadaith"
Are we not using Hazakoth all through the questions of Issurim connected to family? Why should this reasonong not influence us all throughout these questions?
1) The rule of Shenei Kesuvim haBa'im ke'Echad only applies when we would be able to learn one from the other. Here the gemara explains the necessity of both Pesukim, therefore the rule does not apply.
2) We definitely use Chazakah for all paternal family Isurim. Specifically where is it that you feel that the Gemarah should have applied this reasoning? It is only applicable as a Pircha, when we are trying to deduce a Halachah from a non-paternal Issur.
Dov Zupnik