Yeah I don't think their family would be relevant in that case. If the next PM did start supporting Russia then I would criticise them for that decision, regardless of whether family connections influenced their decision or not.
Anyway. Like I say, we don't seem to know whether Starmer's wife even has Israeli citizenship. I definitely think it would be wrong to assume she does just because she's Jewish.
But even if Starmer's wife does have Israeli citizenship, that doesn't necessarily make Starmer pro-Israel. He might be too pro-Israel, but a hypothetical Israeli wife wouldn't necessarily make him pro-Israel.