Sandra is the older twin while Cassandra is the younger twin.

They were adopted by aunt Laila after their parents death.

Their parents died due to a car accident where Sandra and Cassandra was actually involved in. The car accident was so major that Sandra was stuck inside the car for several days with no way out, and Cassandra was passed out due to her head spinning around the car when it fell.

Everyone did everything they could to get Sandra and Cassandra alongside their parents out of the car. They finally got the two out sister's out but the parents didn't survived.

Rianna aka the twins and Paimon's mother took care of the two girls because she was the only adult who actually has kids and is the younger sister of the girls dead father.

Sandra became good friends with Lumine while Cassandra and Aether became well kinda friends cause they don't actually talk to each other.

After Lumine got into a coma, aunt Laila aka Rianna's youngest sister wanted to adopt Sandra and Cassandra. Rianna agreed and gave the girls to aunt Laila.

Cassandra doesn't show emotions cause it hurts her face to even smile or laugh cause there was shattered glasses when she got into that accident. Sandra's hand actually got cut off cause it was stuck under the passenger seat so to get her out, they actually have to cut her hand cause it didn't come out, even if they pull harder, her arm will still be cut off.

Sandra and Cassandra never called aunt Laila, mom, because they didn't want to replace their old mother and aunt Laila understood that and never pushed the two to call her mom, just aunty or aunt Laila.

Also there's anther reason why aunt Laila wanted to adopt Sandra and Cassandra but that'll be a spoiler so I won't tell.