My SO has two sons 5 and 8, the boy's BM has a 12 year old daughter from a previous relationship. SO was her father figure for 5 years, she called him Dad. Now SO and BM have been apart for 5 years and he rarely sees her. She now spends time with her biological father and step-brothers. SO and I have lived together for 3 years and I've seen the girl maybe 5 times. SO talks about how he was her Dad and should get to see her. I tell him she's not his kid and she has a father. We are planning a trip across the country to see SO parents and are bringing the skids to see their grandparents. BM called (two days before we leave) and said that daughter is upset that she doesn't get to go see "Grandma" and "Grandpa" She hasn't seen them in over 5 years and has never said anything until now. SO is now saying we should take her with us! I'm furious! This girl is not my step-daugher, she's not my SO's step-daughter anymore and I think it's a huge imposition for BM to suggest we take her!
Keep in mind I'm trying to convince SO that if he wants a daughter I will be happy to provide him with one.