Formal:
* Ukochana Siostro (pronounced oo-koh-ha-nah shyo-stro) - This is the most direct translation and is quite formal.
Informal:
* Ukochana Siostrzyczko (pronounced oo-koh-ha-nah shyo-st-r-ch-ko) - This is a more affectionate and familiar way to address your sister. It's often used by siblings who are close.
* Moja Ukochana Siostra (pronounced moi-ya oo-koh-ha-nah shyo-stra) - This literally translates to "My Beloved Sister" and is a very affectionate way to address your sister.
You can also use different terms of endearment depending on the situation and your relationship with your sister. For example, you could use "Kochanie" (pronounced koh-ha-nee) which means "Darling," or "Słoneczko" (pronounced swon-ech-ko) which means "Sunshine," if you're very close and affectionate.
Ultimately, the best way to say "Beloved Sister" in Polish will depend on the specific context and your relationship with your sister.