Akpo Sodje won the battle of the brothers and with it inflicted an eighth successive defeat on Pompey.
Glanford Park was the setting for younger brother Sam to go up against his older sibling who had yet to open his account for his new club.
It was the Scunthorpe Sodje, however, who had the last laugh with an 85th-minute winner to break Pompey hearts.
His thumping header from substitute Mike Grella’s cross was his second of the game, having earlier broken the deadlock in the 40th minute.
That was also a header – albeit a free one at the near post, without a Blues player in attendance during a shoddy piece of defending.
Then, two minutes into first-half stoppage-time, Liam Walker levelled with a stunning 25-yard free-kick – his first goal for the Blues.
It stunned the home crowd, who were themselves desperate for a win to ease their relegation worries after only one victory at Glanford Park all season.
In the second half, James Keene and Adam Reed both appeared to have decent shouts for penalties waved away by referee Steve Rushton.
Then up popped Sodje to settle the matter – giving the Iron only their second home win in 15 matches this term.
As for Pompey, it is now 18 matches without victory – just one short of equalling the club’s post-war record.
Blues caretaker boss Guy Whittingham made four changes to the team that suffered a seventh successive defeat on Saturday against Colchester.
With David Connolly sidelined by a hamstring injury, Ashley Harris was pushed into a forward position just behind Keene.
That prompted a recall – and an away Blues bow – for Reed, while Walker was named in place of Shaun Cooper in midfield.
For the 24-year-old it was reward for his impressive half-time Colchester entrance during which his sublime pass set up Keene to score.
Meanwhile, Adam Webster replaced Dan Butler at left-back and Gabor Gyepes started in place of Ricardo Rocha.
Both defenders, who have been struggling with knocks picked up against Colchester, were instead named on a four-man bench. There was no Frankie Sutherland in the squad with the midfield talent called up for Republic of Ireland under-21 duty against Holland.
When the match got under way, Keene came agonisingly close to grabbing an early lead with a piece of superb anticipation.
Walker crossed from the right and when Niall Canavan headed the ball back towards the keeper, the Blues striker had already read the situation.
He managed to lift the ball over the advancing Sam Slocombe but it dropped just over the bar with the net unguarded.
At the other end, the Iron won a succession of corners in the opening stages but Pompey dealt with them comfortably in the damp conditions.
In the 14th minute, good build-up down the left saw Jed Wallace teed up in a central position.
But his powerful 25-yard drive took a slight deflection and flew comfortably into the arms of Slocombe.
Similarly, Michael Collins attempted to net from long distance for the hosts but his angled effort from outside the box drifted over.
It was Simon Eastwood who made the first save of the match, as he thwarted Akpo Sodje in the 20th minute after the striker burst through for a one-on-one.
Karl Hawley should have broken the deadlock on the half-hour mark when Tom Newey swung in a superb cross from the right.
But his header – from six yards out – lacked accuracy and nicked off a Blues player and out for a corner, when, in truth, he should have done better.
The Iron did take the lead, though, in the 40th minute after some poor Pompey defending.
Callum Kennedy crossed from the left and Akpo Sodje was all on his own at the near post to glance a header home.
Moments later the striker should have doubled the advantage after cutting through the heart of the Pompey defence.But he sent a left-foot shot straight at Eastwood.
On the stroke of half-time, Harris found room down the right to fire in an angle shot that was comfortably dealt with by Slocombe at his near post.
Pompey did get their equaliser, however, in first-half stoppage-time through a sublime free-kick from Walker. Harris drew a foul from Canavan 25 yards out and up stepped the Gibraltar midfielder to curl a magnificent left-foot shot into the top corner.
After the break, Pompey had appeals for a penalty waved away in the 56th minute when Keene’s shot appeared to strike a hand, while Harris’ follow-up was charged down.
In the 66th minute, Jimmy Ryan surged forward and unleashed a fierce 25-yard effort which Eastwood appeared to have brilliantly tipped around the post, only for a goal-kick to be awarded. The Iron were increasingly threatening and Collins rode three challenges to burst into the box and tee up Duffy who curled his right-foot effort narrowly past the far post.
It was the Blues, though, who had appeals for a penalty waved away in the 79th minute when Reed tumbled as he attempted to wriggle through a raft of bodies.
Then in the 85th minute Pompey suffered heartbreak when substitute Grella sent a looping ball into the box and Akpo Sodje powerfully headed home.
There was no coming back for the visitors from that galling blow as they suffered their eighth successive defeat.