When Opal is first introduced to Team Avatar she mentions her mother told her many stories about her Aunt Lin, however...
-Su and Lin did not interact for 30 years, thus Su wouldnt have stories about Lin during that time frame
-30 years before, Su was corrupt as shown in Old Wounds, it would be hard for Su to tell a story without making her look bad
What kind of stories did Su tell Opal about Lin? That time I deflected an metal rope and broke Lin's face? When Lin tried to arrest me for stealing all the cookies in the cookie jar? Laughing at Lin breaking up with Tenizen? Beating Lin's ex Boyfriend at a competition of who has the most kids?
I think Su might've had rose-colored spectacles in her storytelling, as long as she wasn't actually /around/ her sister, in the same way childhood stories may be funny despite them being upsetting at the time.
Lin, at least, admitted to something similar about toph - "I thought, 'maybe I should try and reach out to Mom again' ... I [now] remember why we stopped talking".