I don't know about "deeds" (I don't think Lalaith would have
done a lot in the sense of slaying and adventuring), but I think she would make the world a better place.
If Turin had been able to go to Doriath with his beloved sister, it wouldn't have been even half as lonely and sad for him, and he doesn't grow up so dark. He'd still be there when Morwen and Nienor arrive, and they'd be so happy! And probably when having meetings/conferences with their allies in Nargothrond a more normal Turin would fall in love with Finduilas and yay- another man/elf union!
And meanwhile the best best friend of Turin, Beleg, would be hanging around all the time and I think he'd probably fall in love with Lalaith and they'd get married too! (I've wondered about Nienor... Maybe Mablung?)
So once Hurin got out of Angband, he'd have a family waiting for him and he'd want to stick around. Happy ending!
So even without actual "deeds", I think Lalaith just being the delightful little lass that she is would've been the counter to Morgoth's curse. Call her a minor character if you wish, but she may be far more important than you think.
