Rovers player Tomkins - back from a two-game ban for questioning the integrity of a match official - scored a brilliant solo try as he rounded his younger sibling earlier in the game at KCOM Craven Park.
That helped put the hosts 12-2 ahead but Catalans fought back to level and, even though Craig Hall’s 80m intercept put the hosts back in front, the French side rallied to claim a late victory.
Samisoni Langi, the Tongan centre who had a superb second period, dabbed in a grubber kick that saw Lewis Tierney sneak in and beat a flat-footed Will Oakes to the bouncing ball.
It was not an easy conversion attempt for England full-back Tomkins - especially in front of the East Stand that had barracked him all afternoon - but he completed the job to give Catalans their first away win in six months.
It was just what they needed after an embarrassing 46-0 home loss to Salford Red Devils last week but the Robins will know they should have seen this one out.
With a dire first half finishing level at 2-2, they seemed to have taken control with two tries early in the second period.
First, after Josh Drinkwater’s kick forced a drop-out, Chris Atkin’s delayed pass out of acting-half sent Weller Hauraki over untouched from close range for a converted try.
Then Tomkins did his stuff in the 49th minute. Atkin, who made a difference coming on at nine, was again the instigator giving the ex-England second-row room to move down the blindside from 40m out.
But then Tomkins did brilliantly to fend off a defender while holding the ball outstretched one-handed before fooling his brother turning him inside out to reach the line.
Tim Sheens’ side had a stroke of luck, though, when Ben Jullien had an effort ruled out three minutes later.
The Frenchman charged down Drinkwater’s kick and won the race to the loose ball.
Although he was half-tackled close to the line, the back-row stretched out to score - only for referee James Child to rule double-movement.
Catalans were undeterred, however, as they levelled the game with back-to-back tries on the hour.
Brayden Wiliame was too strong for Jimmy Keinhorst out in the centres and, though Tomkins missed the conversion attempt, his side were in immediately after.
Samisoni Langi surged clecar down the opposite flank on the right and Lucas Albert supported for Tomkins to make it 12-12.
Rovers continued to look shaky, no one moving to claim Matty Smith’s high kick and Hall forced into conceding a drop-out.
The KR winger relieved all the pressure, though, with an 80m interception try, picking off Kenny Edwards’ pass.
But he missed the conversion attempt and it proved crucial at the death.
The first half was a scrappy, stop-start and error ridden so it was perhaps no surprise the score finished just 2-2.
Craig Hall and Sam Tomkins exchanged penalties and the latter also saw one effort land wide after the interval hooter but there was little other action of note.
Unfortunate Catalans lost three forwards to injury inside the opening half-hour with both props Remi Casty and Sam Moa as well as No13 Greg Bird going off due to head knocks.
Only Casty was deemed fit enough to return and that was not until the second period.
Rovers got close through Kane Linnett and then had a couple of chances to expose their visitors’ right edge but both times the final pass - via Craig Hall and then Will Dagger - failed them.
Joel Tomkins did think he had got over the line but referee James Child ruled he had fumbled in doing so. The player has clearly heeded his lesson; he did not argue.
However, he was left deflated by the end as his brother stole the show.
Hull KR: Dagger; Hall, Keinhorst, Vaivai, Oakes; McGuire, Drinkwater; Masoe, Addy, Lawler, Linnett, J Tomkins, Hauraki. Substitutes: Garbutt, Atkin, Rooks, Shaw.
Catalans Dragons: S Tomkins; Yaha, Langi, Wiliame, Tierney; Gigot, Smith; Casty, Albert, Moa, Edwards, Whitley, Bird. Substitutes: Bousquet, Baitieri, Jullien, Goudemand.
Referee: James Child (Dewsbury)