Navigating the strained relationship she has with her older sister and sole guardian, a teenager joins the improv group at school as a way to conquer her fears.

Hi meme,

I cannot see the relation between the two sentences in your logline.
You wrote:

Navigating the strained relationship she has with her older sister and sole guardian,

and

a teenager joins the improv group at school as a way to conquer her fears.

The second part of your logline can be an independent logline by itself. Even if it needs more work:
To conquer her fears, a teenager joins her school’s improv group.
Use the above logline as a template and re-write if you think this logline serves the purpose.
But I cannot see the significance of her strained relationship with her sole guardian.
Can you clarify what do you exactly mean by “Navigating”?

I hope this helps
Good Luck