To merge, go to the bottom of the profile page. There is a link that says Initiate a merge. Click on that. Merge the Van Vrankin profile into the Cook profile. After you propose the merge, your cousin will have to approve it.
If you cousin already has his adoptive family attached to him, he will have to change his parents to the Cooks. Then in the bio, he can write that he was adopted by the Van Vrankens and link to them in the story.