Girls we "adopted" in Asia
I can't recommend it enough. Is it hard? Sure! But so is raising your own kids. Is it expensive? Of course. You've added another set of feet under your table. Is it difficult mixing in almost-grown-up personalities you're not used to? That's probably the hardest part. But it helps if you just treat them as your own and set up a few house rules to start with. And you'll probably find the most important rule is to keep their room clean and their own laundry washed. It helps them feel a part of the family to have their own specific chore to do around the house.
I'd say the most important thing you can do as the fill-in parents is to see this time as an investment. Be intentional in investing in this new kid. You may only have a few months to pour love into their lives. And to be honest, you may need some short times away from one another to keep a healthy balance in your home. But our goal was for these kids to know God.
The fun result is your family has some really great--and even hilarious--memories. And you have a deep connection with some almost-related adults later on. You may even get an extra call on Mother's Day!