I just wanted to say. .
Whites, Asians, Hispanics, Native Americans tend to have less kinkier hair than African Americans. As we all know, sebum is a natural moisturizer for the hair, and the more moisturized our hair is the more it tends to grow. Because these women have straighter hair/looser curl patterns it is easier for the sebum to travel down the hair shaft and for their hair to stay moisturized and grow faster.
I'm pretty sure that this contributes to the fact of this girls hair being like it is and growing like it does. She probably has a looser curl which was inherited due to her ethnic background. In regards to her sisters hair, she is NOT her sister and has a whole different set of genes and that is probably why HER hair is like it is.
Her hair to me is average at best. It's plenty of women who aren't "mixed" that have hair that's thicker and wayyy longer than this girls'.
The girl pictured in this video and in one of these pictures is her sister


Relaxers don't have anything to do with genetics.
But, what does your explanation about straighter hair and sebum have to do with genetics considering her hair is straight due to relaxing and heat styling and not genetics...?