Once upon a time, I bought a house so I could scrape the lot and build a custom home on it. The house came with renters, at least one of which was a self-wounding drug addict. It was as sad a story as you can imagine and a sobering reminder of why illegal drugs are illegal. It was a woman who had fled her mother and her husband to run into the arms of drugs. I met both of them, who were desperate to save her. I pray that they eventually did, but having seen that story play out before, I wouldn't bet on it.
In the four months before we bulldozed the place, the renters stayed. We got a total of one month's rent out of them. They knew they didn't have to keep a relationship with us and they were going to have to leave anyway. Their credit ratings and our references for future rentals meant nothing to them, so they simply occupied the house. This being California, there wasn't much we could do. The eviction process is prolonged and expensive for a landlord. In the end, we decided to remain friendly and just let them know when the place was going to be demolished. They left a couple of days in advance of that.
Had they wished, they could have stayed a few more months and thrown a huge wrench in our schedule. If someone lives in your house and claims it as their residence with no other place to go, they are considered tenants in California with all the rights thereof. That means you need to go through the full eviction process to get them out. When you read stories of adults living with their parents and wonder why the parents don't just kick them out of the house, that's part of the reason why.
An eviction process takes months. Just getting to the point where you can file papers is a month. Imagine what it would be like to live under the same roof with someone, particularly one of your children, who knows you dislike them so much that you'd throw them out in the street. If they're desperate and/or addicts, things could get really dicey. If you're elderly, fearing for your safety would be natural and sane.
That's something to consider when you let Junior move back in with you.