It's impossible to ask this question without spoilers.
In Episode 3 we learn via a flashback that;
The opening is a flashback to Dr. Smith's past, revealing her real name: June Harris. Her sister, an accomplished businesswoman, offered to leave all her possessions to June as she left for Alpha Centauri. June, a petty criminal and family Black Sheep, drugged her and stole her identity, leaving her behind instead.
Why would she do this? Did I miss something later in the episode or series that explains it? As far as I can see the disreputable sister is being offered the chance to take possession of the "good" sister's life on Earth. Her house, her possessions, even if this doesn't include her bank account (gotta pay for that smuggled whisky somehow), it looks as if June would be set up for life just living off this stash. Why would she throw this away for a life pretending to be someone else, knowing it could all go wrong if she's found out? Why not take the fresh start on Earth?