So it is a sports car and your cousin has a small saloon type car. Your fuel costs are going to be more than his. Perhaps, but so what?
Well, the RAC fuel cost calculator uses the make and model of the car to calculate fuel costs so if it said it would cost £127 to do 701 miles, that means you car has an mpg of about 30. His saloon car probably has an mpg of at least 60mpg, most likely even more. The hire car probably has a similar mpg.
Therefore:
Separate travel
Your cost in your car = £254
CF cost in their car = £127
Hire Car
Cost of hire car and petrol = £247
Split 50:50 that's £123.50
You save £130.50
They save £3.50
Split in 3 that's £82
You save £172
They pay £37 more than travelling in their own car
So it would cost them £37 for the privilege of taking one extra suitcase. Ryan Air don't charge that much! Yet you are getting 68% saving on what it would cost you travel alone. Is that fair?
Presumably now they aren't doing a 3 day journey with stop offs to sightsee they don't need so much luggage so they probably don't even need that extra suitcase.
I'm not sure I would fancy sharing a car with a toddler on that journey but, on the other hand, you said yourself you enjoy your cousin's company despite his cheeky fuckery and you've done yourself out of what could have been a fun short break. And depsite his track record, I don't think he was being that cheeky this time once you look at the numbers.