As the person above me has said "white knuckled" IS just an expression.
Here's what comes up on google when you search "white knuckled grip meaning":
Even I, as a white person, don't have white knuckles even when I grip tightly and I'm extremely pale. In fact, if you DID have completely white knuckles I would be low-key concerned.
While I understand where you're coming from, please look it up before you start accusing people. It's definitely not fun to be discriminated against or forgotten about. While there isn't anything wrong with using this particular phrase there might be a problem somewhere else, and for that, I recommend contacting the devs on their discord server if you can. Respectfully of course.
As for the different mother part. If you choose to be human you will have the same mother as your cannon sister, if you choose a different race you will have different mothers. BUT even when you have the same mother the physical traits that are true for her don't HAVE to be true for your character. Even if siblings don't look anything alike they can still have the same parents, they don't even have to look like their parents and they can still be related. Genetics are a complicated thing.