An important reminder as to the complexity of the current events on Egghead, Sentoumaru was previously depicted as having some kind of relationship with Kizaru. He addressed him as Uncle iirc, though I don't know if there was any indication whether it was a literal and actual bloodline relationship or else a mentor-disciple relationship considered spiritually similar.
Regardless, we can assume that this makes the question of whether Kizaru just outright attacks Strawhats and aids the WG a little more complicated. He may end up simply functioning to rescue Sentoumaru.
I don't know, again, how strong the relationship of those two are or what it means to him so the ball is in Kizaru's court in how he reacts to this. Will he maintain total loyalty the WG and assist Lucci and the others? Will he now consider Sentoumaru an enemy? Will he be confused and believe that it was Strawhat's crew that attacked Sentoumaru and initially attack them? Will he perceive what actually happened and be enraged against Lucci and WG forces for Sentoumaru's sake?
I am 100% certain that Sentoumaru being on this island and getting attacked plays a role that is meaningful - it has a purpose outside a mere kickback to Sabaody.