I think I started a topic like this before but I didn't much response out of it. I will try again and this time hopefully with some help. You see when I started the Heatherwoods I did expect it to be permanent this was to my exact understanding. The fact that my siblings don't show up as a sibling, well I think that may very well be a bug. For example I have done:
family relation ashaia

Ashaia bears no relationship to you.

family relation janise
Janise bears no relationship to you.

family relation elyse
Elyse bears no relationship to you.

Also when I got to family tree Heatherwood it shows up to something like this.
************************************************
Active Family Members: 5
Inactive Family Members: 1
---------------------------------------------------------------------------
Progenitors:
Founder: (A) Mistie Heatherwood
Founder: (A) Dysolis Heatherwood
(I) Caelyn Kinsari
(A) Elyse Heatherwood
(A) Janise Heatherwood
(A) Ashaia Heatherwood
All showing up on the family tree and are considered a family member. I think this really is a bug but I am quite unsure and I don't want to report bugs without asking beforehand.

When I found my parents and after the consideration process, I did not show up on the list just like you described. I had to go and family acknowledge at the hall of records and pay 10k gold. Then everything was like it should.