Question
Is the concept of sin (papa) developed in Rig Veda?
Answer
The concept of Sin is already present in Rig Veda and other answers are explaining that too. But as you are specifically looking for the sin of type as incest, so it is also present in Rig Veda, Mandala 10, Sukta 10. In this Sukta Yami is convincing Yama (her brother) to make him as her husband but Yama refused because for him it was a sin.
न वा उ ते तन्वा तन्वं सं पप्र्च्यां पापमाहुर्यःस्वसारं निगछात |
अन्येन मत परमुदः कल्पयस्व न तेभ्रात सुभगे वष्ट्येतत ||12 I will not fold mine arms about thy body: they call it sin when one comes near his sister. Not me,—prepare thy pleasures with another: thy brother seeks not this from thee, O fair one.
Rig Veda, Mandala 1, Sukta 190 also mention the word Papa or Sin.
ये तवा देवोस्रिकं मन्यमानाः पापा भद्रमुपजीवन्ति पज्राः |
न दूढ्ये अनु ददासि वामं बर्हस्पते चयस इत पियारुम ||5 Those, God, who count thee as a worthless bullock, and, wealthy sinners, live on thee the Bounteous,— On fools like these no blessing thou bestowest: Bṛhaspati, thou punishest the spiteful.
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