elder climbing into younger's crib

The past couple of mornings, we have found our 3yo in the crib with her 18mo brother. They are both awake and playing happily. Are there safety issues that we should be worried about or can we just let this continue? The younger is still in a sleeping bag, and I don't think the older is likely to try to climb out [she's not that kind of kid]; the younger actually outweighs the older, so I don't think we need to worry about her smushing him.... B/C financial crisis, we are currently in a position where we may change jobs/cities or not within the next few months, so we have decided to keep the younger in the crib for the time being rather than transitioning him to a bed [another piece of furniture to buy/move at a time of uncertainty....]