I have had to post on this thread, I hope I don't upset anyone - I really don't mean any offence to current foster carers.
I am the natural born child of a foster carer, and I have to second a lot of what therhubarb says.
Put your own child first - no matter what. You are exposing them (especially when they are older) to all sorts of outside elements that you would normally shield children from (neglect, abuse, violence, paedophilia) and you have to expect that to have an effect on them to some degree.
I was sexually abused by one foster brother, have been beaten up by others, had my toys as a young child broken / stolen / taken from me for fun. Fostering has literally torn my family apart - my eldest brother no longer speaks to my mum, and it has taken years of therapy for me to have a relationship with her.
Fostering is amazing when it goes right, but not at the sacrifice to your own children - and I think when you are in that situation its hard to keep them at the fore front of your decisions as you have other peoples needs to consider too.
Also, having a sibling is completley different - they are there to stay, so no constant change in your house of people coming and going, getting attached and having to say goodbye etc.
Again, I hope I haven't upset anyone, but I couldn't not post.