SIA:
I see Sia's hastiness to react to emotions of the heart (where Anand is concerned) as probably mirroring that of her father in his younger days. Remember in the beginning, he was quite passionate for Simran, but that fizzled out when Kiran came into the picture. We see Sia adamant about her feelings and taking the initiative to reach out to Abhi, whereas Simran would have adopted a more passive approach and let the person of her interest come to her.
As far as her logical thinking and sense of reasoning, she has the qualities that Simran has portrayed. I think the storyline mimics the past of the older core characters. In the past, we see the love triangle with Neha, Simran and Abhi, where Simran unknowingly causes Neha to get jealous. In the present, we see love triangle with Sia, Astha and Sid where Sia unknowingly causes Astha to get jealous.
(I would like to say though that for me, the actress who portrays Sia does not appeal to me as much. She does not have a strong likeable presence where the audience immediately embraces her. I think they could have done a better job with finding another actress for the role).
ASTHA:
Astha definintely has inherited all the insecure feelings that her mother had. Despite being showered with love her whole life, she is not confident in herself and in is quick to loose trust of those around her. She appears to be loving and cheerful in her comfort zone, but outside of that, she is terribly insecure and reacts without consideration.
I think she inherited that part of her that does not care the consequences of her actions, from Abhi. Even in the past, when Neha was disturbed and hurt, she would bare her burdens silently, or talk it out with her mother or friend. Astha simply reacts first, lashes out and hurts others. Abhi was very much the same by hurting Simran and his father is different instances.
(My opinion of the actress who plays Astha - I think she conveys the character of the person well. She comes across as being spirited, spoilt, quick tempered, insecure etc...It is obvious that her character will experience an 'awakening' at some point. It will be interesting to see how she handles the maturity of that role.)