Unnecessary `return`.  Last expression of a method in Ruby is the return of that method, so you can just use `nil` here, although I'd prefer single `return` code, so:

```ruby
def get(sid)
  session = nil
  sid = sid.to_i
  
  if sid > 0
     session = self[sid]
  elsif sid == -1
    sid = self.keys.sort[-1]
    session = self[sid]
  end
  
  session
end
```