There is also the issue of when exactly you know if you have accomplished the goal of being a "forever family." Is it like marriage where you only know if the vow of "till death do us part" has been upheld by one of the parties dying?
I imagine an adoptive parent or professional created the term "forever family" to offer reassurance to adoptees and their families. I can appreciate the intention but worry that the term reassures the adoptive parent more than the adoptee. Stating you are a "forever family" is like someone saying s/he is going to be honest with you. It raises suspicions and doubts.
Adoptees tend to be sensitive about promises, lies, secrets, and honesty. Adoptees appreciate the truth because it validates their experience and honors their intuition. Don't promise an adoptee something you can't deliver. A parent can't guarantee that s/he won't die or won't get divorced. The world is filled with random events and situations that are out of our control. Can a parent promise a "forever family?" Not really.
So what can an adoptive parent do?
Instead of saying you are a "forever family," be one.
Show adoptees that they are valuable members of the family.
Be interested in who they are and what interests them.
Prove that family is important by spending time together and appreciating what each person brings to the family.
Acknowledge the importance of birth family connections.
Let adoptees know you love them even if you aren't thrilled with their current behavior.
Adoptive family relationships are built upon shared moments and daily interactions. Trust takes time, a very long time for adoptees. Allow adoptees to gather family moments, piece them together, and draw their own conclusions. Hopefully with the passage of time and a collection of consistent moments, the adoptee will feel like s/he is in a family that will be there for her/him... forever.