I thnk you can achieve that via javascript. Check how phoenix creates inputs and add a event click listener to a button and append child to these to increase numbers each time. Also add |> cast_assoc(:siblings)in your changeset after cast.
For example:
<input class="form-control" id="user_siblings_1_name" name="user[siblings][1][name]" type="text"> <input class="form-control" id="user_siblings_1_surname" name="user[siblings][1][surname]" type="text">
