Although Eilis sees Rose's clothes as beautiful and glamorous at first, that vision changes greatly after Rose's death.
At first, Rose's clothes represent all the glitz and glamour that young Eilis associates with her big sister. Rose is one classy lady—she plays golf like a fiend, parties like a rock star, and breaks hearts like Taylor Swift—which naturally means that she also sports some top-notch duds. Before she moves to America, Eilis would give anything to have clothes as nice as Rose's.
That all changes after Rose's death. By this time, Eilis has been in the States for almost two years, which is an experience that changes her greatly. In more literal terms, however, it also means that she now has some amazing clothes—clothes even more amazing than those of her big sister. Still, Eilis' mom tries to make her to wear Rose's clothes, but Eilis refuses because it makes her feel like "Rose's ghost, being fed and spoken to in the same way at the same time by her mother" (4.102). In a way, it's almost like Mrs. Lacey is trying to transform Eilis into Rose.
But Eilis isn't down with that—she's worked really hard to get comfortable in her own skin and isn't going to let anyone take that away from her, even her mother.