If this really is an honest investigation of a crime committed by the Bidens, where are the indications that a crime may have been committed? Before opening a homicide investigation, the cops usually need to know that a) someone has died, and b) it wasn't by natural causes. I haven't seen any of that from Trump, et al., just a "Biden's son was on the board of _company_." Did the Ukraine or that company get any special treatment during Obama's presidency? I haven't seen mention of any on the news. That doesn't mean there wasn't any, but Trump needs that before beginning an investigation. Friends and relatives of a new [Vice] President still need to be employed so they can put food on the table. In general, the friends and relatives aren't just going to quite because someone was elected. I could imagine a case where there is a conflict of interest, but it would be something like New President's Sibling happens to be high in the Chinese government or military.
Further, when the cops do their investigation, they keep detailed notes and have chains of custody on evidence, allowing the defendant's defense team to question the cops and verify their work. Said notes and evidence are also needed if Internal Affairs opens an investigation to verify the cops are doing their jobs correctly. Trump is preventing the "cops" from talking to the "IA investigators", that being Congress.