Give you some context to understand these two words. 1. Formally people use jiě jiě to introduce or tell a fact. zhè shì wǒ jiě jiě。This is my older sister. wǒ yǒu yī gè jiě jiě。I have an oder sister. 2. To use within in the family or with close friends. wǒ shì nǐ jiě。 I'm your older sister. nǐ jiě ne？ Where's your older sister? 3. To make it more amiable people use jiě jiě. For example the hostess in a children's show called yàn zǐ jiě jiě. To show more respect people use jiě after their family name to call older female or supervisors at work. For example zhāng jiě, lǐ jiě.

Thank you for your answers. Let me tell you you are using the comment option instead of the Answer option so I cannot point out the best answer.

no much difficult. If express "my sister" in spoken Chinese, we usually say "wo de jiejie（我的姐姐）"、"wo jiejie（我姐姐）、"wo jie（我姐）,we usually don't say "wo de jie"(我的姐)". Usually children say jiejie and adult say jie,but this is not very strict.