That's true, and seems to be the most likely answer. We saw the effect once before when Alto still had one of the earrings (that Grace/Mophead somehow reobtained). The end of Sheryl's farewell concert where Sheryl and Ranka both sang Infinity and that allowed Alto to escape death and save Luca.

But I suppose the part that remains a mystery is which of the scientists gave them to her and why she cherished them so much. Sheryl certainly doesn't like Grace now, but even in the beginning it seemed like Sheryl did not view Grace in any special way outside of being her very useful, competent and multi-functional agent.

Since the scientist and project leader Mao Nome adopted her, she undoubtedly treated Sheryl in a way that a gutter orphan never would be. Going from eating garbage to gaining a kind mother-figure and being treated well (all the researchers in the picture from eps 18 are female) would probably make quite the impression on her. It also seems like Mao Nome was one of the oldest researchers, so she may have died. Sheryl would certainly keep them safe if someone who was very much her mother/grandmother gave them to her. If Grace had an alterior motive out of it, then all the better for her to stay close to Sheryl.

In fact, it's possible that Mao Nome was THE Mao Nome (from Macross Zero). The research project took place in 2053, which would be 55 years after Macross Zero took place (since Macross SDF-1 starts 1999). From the looks of the photograph, 10:16 in eps 18, the ages would be about right for that. It also looks like Ranka's mother was there...a Ranshe Mei.

To Sheryl, they may just have always been a keepsake. To Grace, they were the beginning of her cadre's plot.