My understanding is that his wife is Jewish but that doesn't mean she is Israeli. I think she's just British. It's like how not every Muslim is Asian; there are British Muslims.
It's fair to criticise Starmer for his policies on Israel, obviously, and it's fair to say he's too pro-Israel if you think he is. But I don't think his wife's background necessarily determines Starmer's views. Just like it would be wrong to assume that Sadiq Khan practices political favouritism towards Pakistan just because he has family from there.