I believe it's easier to break this down into two parts: how does Lenny add to George's regression, and the same for Curley's wife. I say "add" to his regression, because it isn't certain whether or not Lenny or Curley's wife absolutely causes George's regression, rather, would George have spiraled down into some other disastrous end. Keeping this in mind...
Lenny certainly hinders George's progress in life first and foremost because of his social/mental disorder. Lenny's inability to take responsibility for his own actions, let alone even survive on his own, forces George to assume responsibility for them both. However, it isn't simply because George feels obligated through some higher order thinking or moralistic vew, rather because his own prior actions (pushing Lenny into the river) prompted a sympathetic, perhaps paternal/fraternal response. This, coupled with Lenny's Aunt's request, provide most of George's motivation for keeping Lenny around. As such, George must consistently monitor Lenny's actions, inhibiting his ability to move freely and seek more permanent employment, as far as the Depression would allow him.
As an aside, though, while Lenny hinders George's progress, their relationship, and socioeconomic status, are rooted in Lenny's ability to haul bails of hay. Without Lenny, George may not even have received work during the Depression.
Yet, the question about Lenny's hindrance remains. Similar to Lenny's inability to assume responsibility for himself is his inability to discern personal boundaries. His interactions with Curley's wife, the "girl up in Weed," and even George's anger illustrate how poorly Lenny can function in society.
Similarly, Curley's wife's attributes present fitting variables for Lenny's social faults: soft hair, beauty, coquettish personality. Because Lenny is unable to resist these qualities, and because Curley's wife moves about the grounds freely, George is again consistently tasked with watching out for Lenny.