I don't agree.
Przybrany refers to something "taken as your own", "assumed", it's not a legal term, it's not restricted to parent-child relation, it can be also used in other contexts, like "przybrane imię" (about a pseudonim/nickname).
"an emotive element of alienness"
- it's too subjective to decide. In my opinion a sentence like "My stepmother is leaving for holiday" conveys absolutely no information about your emotional stance towards your stepmother.