When their son takes his own life, his determined mother must seek out her orphaned nephew in an effort to support his dreams like she never could for her own child.

mrliteral:
>>>Never start a logline with ?When.?

Why never?? Would you care to explain?

Never start a logline with "When." No one cares about things that happen; we care about the people to whom things happen. Start with the protagonist.

Other major issues have already been mentioned here: use of "their," why is her quest a must...also, if her nephew is orphaned, why does she not already have custody? Is the death of her son and those of the nephew's parents related, or concurrent? If not, where's the other kid been all this time? And what do his dreams have to do with anything? What's really at stake here?

A logline should be a compelling summary of the script, highlighting these basic details: protagonist, antagonist, conflict, stakes. Give us this info, and phrase it so it flows in one smooth sentence.