Mrs. Lacey and Eilis don't exactly have the healthiest mother-daughter relationship in the world. They love each other a great deal, true, and Mrs. Lacey is sad to see Eilis go to America, but their lack of communication sometimes places undue strain on their relationship. Whether you attribute this to her being a widow, or simply due to the fact that she misses her kids (who are scattered across the world), this emotional barrier is an obstacle that Eilis must overcome.
For example, Mrs. Lacey's overwhelming presence can sometimes makes Eilis feel "like a child when the doctor would come to the house"(1.186). In fact, Eilis later realizes that she and her mom never really spent time alone together, with Rose acting as a buffer between them. That protected Eilis in many ways, sure, but it also left her unprepared to deal with mommy dearest after Rose's death.
This leads to a huge mess when Eilis returns to Ireland. Mrs. Lacey is clearly still messed up by Rose's death, having kept her room perfectly intact and forced Eilis to wear her sister's clothes. She even gets her a temp job at Rose's old office. This weirdness really throws Eilis for a loop, making her feel like "she was Rose's ghost, being fed and spoken to in the same way at the same time by her mother" (4.102).
That might sound creepy, but you should cut Mrs. Lacey a break. Her eldest daughter has just died, her sons are off in England, her husband passed away, and her other daughter lives in Brooklyn—so pardon her if she's not in tip-top mental condition. What's more, Mrs. Lacey does prove that she wants what's best for Eilis by giving her daughter her blessing to return to America, knowing that Eilis' future will be brightest there. Although that's a big sacrifice to make on her part, Mrs. Lacey would do anything for the people she loves.