Why are Theo and Tabitha Galavan of different skin color?

It is possible for siblings to have widely different genes including genes for appearance.

Suppose that a black person and a blonde Scandinavian have children. Their children are likely to took mostly black because darker hair, skin, and eye colors are usually dominant over lighter colors. But those children may have recessive genes for lighter colors.

Suppose that another black person and another blonde Scandinavian have children, that also look mainly black but may have recessive genes for lighter coloration.

Remember that each person has two sets of chromosomes, one set from the farther and one set from the mother. When that person's body makes reproductive cells, the chromosomes are mixed up to one single set of genes, some from the mother and some from the father. And each single reproductive cell, egg or sperm, has a separate random mix of chromosomes.

And when a sperm cell fertilizes an egg cell the fertilized cell now has two sets of chromosomes, one from the father and one from the mother.

So if a child from one mixed relationship and a child from the other mixed relationship have children, about one quarter will have "black" hair color genes, two quarters will have mixed hair color genes, and one quarter will have blonde hair color genes. And the same proportion for various other genes including other appearance genes.

So it would be perfectly possible for the two mixed race parents to have at least one child who looked black and at least one child who looks blonde, blue eyed, and pale skinned. The more children they have, the more likely they are to have at least one of each of the extremes of appearance.

Thus it is possible for full or half siblings to have different racial appearances.

in the Disney TV series Pair of Kings 2010 to 2013 the protagonists were fraternal twins, one who looked "black" and one who looked "white". No doubt their parents were both of mixed race.

And of course adopted siblings don't have to be of the same race.

There is even a phenomenon called superfecundation, where fraternal (not identical) twins have different fathers, which has ocassionally happened among humans.

https://en.wikipedia.org/wiki/Superfecundation1

I remember a case in Germany where a women who had affairs with American servicemen gave birth to fraternal twins, one "white", one "black".

Maybe Tabatha is half white. They could be half siblings. They could have the same white father but different mothers. Tabatha's mom could be black.