The bulldozing Saracens forwards played their first match together since Billy's summer arrival at Allianz Park when the London club launched their Premiership title challenge with a 42-20 rout of London Irish at Twickenham.
It was out of family loyalty that the 20-year-old left Wasps, but the brothers of Tongan descent remain fiercely competitive.
While Billy was celebrating winning his first caps for England on the summer tour to Argentina, he watched Mako steal the limelight by making three Test appearances in the Lions' series triumph over Australia.
"I was a bit envious when I watched Mako in the summer. He did well and I'm very proud of him, but obviously I'd have loved to have been the centre of attention in my family," he said.
"Mako always seems to do things before me. I'm always playing catch up. Maybe I'll be the first of us to get a girlfriend!
"Whenever I beat him on FIFA or Rugby Challenge on the PS3, he likes to smash a few controllers but I try not to egg him on because that temper will come on to me.
"I try to stay quiet when I lose but when he wins he can't get enough of it.
"We live together and I find myself being a bit of a maid. Not because I'm being forced to, but I like to be clean.
"I don't fear him, I respect him because he's the elder brother and what he says goes.
"If he tells me to take the bin out, even if I'm tired I still have to do it.
"It's just the way we are. I don't mind it. I'd rather be like that than when he asked me to do something I just answered back."
Vunipola, one of the summer's marquee signings, has lost 5kgs since switching clubs and, on the evidence of his performance at Twickenham, both Saracens and England are set to benefit.
Selected at blindside flanker instead of his more accustomed position of number eight, he battered Irish's defence throughout and the Exiles must have winced when they saw Mako arriving from the bench to join him in the second half.
"It was pretty fun, it was good to get a win, start the season on a good foot and to play with my brother which was cool," Billy said.
"I've not had the experience of playing alongside my brother too often, it's been a couple of years. It's fun and enjoyable.
"We give each other little pats on the back and keep each other going.
"We don't chat so much, it's more gestures - eyebrows and slaps on the bum. It keeps us going."
- Sports & Recreation