They would have been more senior, but Parliament made the decision based on more than just seniority. Sophia's mother was the proper starting point, from a legal seniority perspective, but Parliament took great care to ensure none of the heirs, or heirs of the heirs, would be...problematic. And Sophia's family was very very problematic.

Some of them were frankly just dead. Or their children were dead. Or their children's children were dead. This was not terribly uncommon. Sophia's oldest brother drowned as a teenager in the 1630s. Her brother Maurice also drowned while sailing to the Virgin islands in the 1650s. Her sisters Henriette Marie and Charlotte died young as well, although not via drowning. Her sister Elisabeth became an abbes, not exactly a recipe for producing heirs. All died without children.

Others still had not inherited titles of their own, like Sophia's brother Rupert, or rather, his children. Rupert's only surviving daughter was a title-less bastard, so she's out too.

Naturally, Catholics, those with Catholic children, those who married Catholics, or those with children who married Catholics, were right out. Whenever possible, they wanted the heirs' families to be totally free of Catholicism – look both back to their parents, and forward to their children. Her eldest brother, Charles Louis I married his daughter to a French Catholic, possibly opening the line of succession up not just to Catholics, but to French Catholics. And we can't have that. Sophia's sister Louise Hollandine ran away from home to become a nun in France, and her brother Edward converted soon after, so the both of them and their children were extra double out.

Some of Sophia's siblings had...problematic histories. Philip Frederick, her brother, was accused of having an, ah, intimate relationship with his mother. And his sister. Naturally, Phillip up and murdered the exiled French military officer who accused him of incest. Instead of facing trial, he ran off to be a mercenary. Although, to be fair he died childless way back in 1650, it goes to show Parliament didn't exactly have the pick of the litter all the time.

The last candidates (aside from Sophia) were the descendants of the aforementioned Charles I Louis, Sophia's older brother. Charles I Louis himself died in 1680 and most of his children were the product of morganatic marriages – meaning their mother was of lower status and their children were ineligible to inherit anything.

Charles I Louis had only one living heir who could legally inherit titles: Elizabeth Charlotte. As I mentioned before, Elizabeth Charlotte was married to a Catholic, so she's still out.

All that really leaves us with is Sophia. But that's not to say that Sophia didn't have anything else going for her at the time. She was raised in the Netherlands and spoke Dutch, which King William of England quite liked. Her son and heir George (later George I) was a good Protestant, and he (used to be) married to a good Protestant. They had a son who would later go on to be a good Protestant (later George II). The rest of Sophia's children were also Protestant, save one son who surrendered his claim without a fuss.

So Sophia was rather far down on the list of seniority, but between death, bastards, and Catholicism, she was far and away the best choice.

Unfortunately, I'm not aware of any comprehensive history on Sophia's family. I had to do a fair bit of one-off reading to get some of these details. Her brothers Rupert and Maurice both participated in the English Civil War, which most books by Christopher Hill are solid bets for more info. Brendan Pursell's biography of Sophia's father, Frederick V titled The winter king : Frederick V of the Palatinate and the coming of the Thirty Years' War focuses mainly on the 30 Years' War, but does mention some of his children. Louise Hollandine, being an artist, runaway, nun, and all around interesting figure has been the subject of a few articles with this one being the one I used.

edit: touched up sources