"How are you, Eilidh and Elizabeth?"

Translation: Ciamar a tha sibh Eilidh agus Ealasaid?

[HoldenChance]

Is "sibh" the formal/plural form for "you" ? Id so, would "thu" be the informal/singular form?

[silmeth]

Yes.

This is explained in the tips and notes to the lessons, especially to the Feelings skill.

You can access the notes on the web browser version of Duolingo at https://duolingo.com and at the https://duome.eu/tips/en/gd website. Unfortunately the tips might be missing in the mobile Duolingo app (in that case just open your browser and log in to duolingo.com for reading).