Well, one challenge is your children by your marriage. If those kids don’t know about the illegitimate child from her teens, how are they going to react to discovering a big brother/sister?
A child given up for adoption deserves to know where they came from. The younger siblings feelings are not paramount. 5th h
It often doesn’t go well when your kids find out - that is why a lot of unwed mothers “live with it”, keep it a secret, and don’t tell (hubby/kids). Often, they are not agreeable to having the intrusion/upset in their lives of the child given up for adoption making an appearance (phone call, letter, in person) one day.
This is why I remarked that the situation I described ended up a very happy reunion. The mother had been haunted for years about the little girl she gave up hours after birth & had searched for her - wrote a letter to her every year on her birthday, sent to the unwed mother’s home (they did not pass them on to the adopting parents). Her kids did know there was a child outside of marriage prior to their father. They have been very kind and welcoming, which was quite a blessing because it easily could have been the opposite. Each circumstance is unique.
Exactly that happened in our in-laws’ family two years ago. The long-lost son was received with open arms by the entire legitimate family. Really amazing. Humans have the ability to be sensitive and accepting — sometimes.