A part of Amelia is very jealous of Angela, not precisely because she is getting stronger, but mainly because she is doing it so well with Lucien.
But both herself and Envy couldn't help but be impressed by the speed that Angela reached the second layer the Mortal Realm from the first. Of course, if Lucien wasn't lying, but he has no reason to lie.
While Envy wants more and more to have Lucien under her control, Amelia began to think that he can fulfill her desire to become stronger than their sisters faster than Envy can; after all, it is obvious that Lust's powers are better at helping others while most other sins, like Envy, focus on strengthening only them and their hosts.
Amelia is also much more inclined to believe in Lucien, her brother, than Envy, a manipulative demon, even though the soul contract prevents them from harming each other.