The IRS looks at the "purpose" of the "trip." If the main purpose of the trip is family, and you spent an hour doing a shop, they will not likely allow the mileage that will make the shop a very losing proposition.
My personal policy is that unless the shops produce enough cash flow to cover the gas, I do not count the miles I drove to get to my personal errand, but only the additional miles to get to the shop. On my spreadsheet this is "shop miles" in one column and "anyway" miles in the other. If I am doing seven mystery shops in Phoenix and run by my dad's for an hour since I'm already in the area, then only any additional miles I drove to get to my Dad's are not counted; the rest of the trip is business.
If I fly to Italy in October like I'm planning and pick up a mystery shop in the Heathrow airport on the way, you can bet everything you own that they will not let me deduct the airfare to Heathrow as a business expense against the $30 I made in the airport.
That's an exaggeration, but your 150 mile drive is the same situation.
Time to build a bigger bridge.