would あなた belive me if i 発言しました i was related to grace kelly?

Only if あなた can answer the 質問 ...
How?

No. Though i'd still be jealous with every fibre of my being just in case あなた were speaking the truth.

I would ask for lineage tracing. It wouldn't have to be fancy または expertly done, but enough to prove that あなた are indeed related.