I had requested for special dua for my sister's marriage proposal. We got a one proposal for my sister, everything seems fine except boy looks and my sister is not agreeing with this proposal because of boy looks. But my parents have agreed as they said looks doesn't matter. My father asked my sister to say yes for this proposal. But she refused and not happy. What should my parents do in this situation? They are so worried because after a long wait we got this proposal. Considering the situation, can my parents go against my sister's rejection? And say yes to this proposal.

No. They can’t, and if they did, the nikah is void.