A soft-hearted young woman determined to find her birth parents stumbles on a deadly alien species that may hold the key to her origin.

In the revised version of the logline the death of the stepfather is described as the inciting incident. However, I don't see how that would necessarily motivate her to do so, she knew that her biological parents are somewhere away from her before he died so why is it imperative she find her biological parents now?

If she is unable to take care of her self and needs a guardian, then what makes her think she can go on the journey to find her parents? The logic doesn't add up in this instance. However, if she were to be young enough for the authorities to put her in foster care, and in order to avoid this she seeks out her biological parents then her motivation is clear.

The logline would benefit from describing the inciting incident. If you make clear the event that motivated the character to need to find her parents, then you dramatise the situation and provide the audience with an opportunity to develop empathy. It becomes personal and a 'must do' goal instead of the MC deciding on a whim to seek them out.

Your logline doesn't read like it's complete. Instead it seems as though you are just giving us the inciting incident then not telling us what she must do about it.

"When she discovers a deadly alien species that may hold the key to her heritage, a softhearted woman must..." (Then tell us what she must do)