In Star Trek Nemesis, when the solar system that Romulus and Remus is in is shown on screen, Remus is referred to as a planet and also looks that way on screen; but when Shinzon is telling his story about being on Remus, the flashback shows Romulus being in the background behind Remus like it is a moon to Romulus.
Romulus has two unnamed moons and Remus has no known moons and is tidally locked with its main sun, Remans live on the night side.
The two planets do appear to be close to each other in shots, and in this picture from Memory Alpha:
This might make sense if they are both in the "Goldilocks zone".
First off, Data outright says that it's a planet during his briefing to Picard:
On top of that, the official novelisation for the film "Nemesis" makes it abundantly clear that Remus is indeed a planet, rather than any sort of planetoid or moon.
Romulus and Remus-twins, but not equals, for while Romulus enjoyed a regular night and day, Remus was placed too close to the sun. Half of the planet baked constantly, a sere, unlivable desert; half remained in constant night, and on that half, the population dwelled, and over time had evolved into hideous, light-blinded creatures. The Remans had already proven themselves inferior by living for centuries as slaves under Romulan rule. Had they been worthy of the rights and status accorded Romulans, they would have fought for them.
The script also offers this information
EXT. REMAN HOMEWORLD - NIGHT
On a stark, desert planet with monolithic mountains and harsh crags shooting upward. The only light in this dark netherworld comes from the flames that accompany the hellish mining operations everywhere around us.
INT. SENATE CHAMBER - NIGHT
We observe the Romulan crest, an imposing bird-of-prey clutching a planet in either claw, not unlike the KAOS emblem from "Get Smart," that dominates a wall. As we hear:
SHINZON (V.O.): Consider it The great symbol of the Empire... But the bird-of-prey holds planets. Romulus, Remus. Their destinies conjoined...
And then we see him... SHINZON!
SHINZON: Yet for generations one of those planets has been without a voice. We will be silent no longer.
Perlman: The Viceroy and he met in sort of a mining situation on the planet Remus. The Viceroy is Reman, and [Shinzon] was a young boy at the time. We were exiled to this mine on this dark planet. I don't do well in light, but I took some sort of control over his development, [his] evolution into this man who eventually came to power over Romulus and Remus... He's very mysterious - he's like an iceberg, you can only see one-eighth of him and I like that, playing what's not seen and what's not explained. The script [by John Logan] is really good, written by a world-class screenwriter. It's not predictable, it's not obvious, it's very well-realised. That, to me, is key in developing enthusiasm.