On the Parliamentary Roll, c. 1312, there is ...
1) Sire Rog'r de Mortimer (Mortimer-77)
... and ...
2) Sire Rog'r de Mortimer le oncle (Mortimer-153)
On the Stirling Roll, c. 1304, there is ...
1) Sir Roger Mortimer (same coat of arms as "le oncle")
... and ...
2) A John, his brother.
On the St George's Roll, c. 1285, there appears to be ...
1) Roger Mortimer-605, father (recently dead)
2) Roger Mortimer-153, son
3) William Mortimer-606, son
4) And a John de Mortimer, with the same coat of arms as the first Roger.
In, or about, 1285, this could possibly be the father's brother (don't know anything about him, including death date), or the grandson John who died about 1284 in a tournament.
We have Roger Mortimer-77 with a brother John, that dies in 1284, two decades before the Siege of Stirling.
We have Roger Mortimer-81 with a brother John, but they're both born too late, and John dies young.
We have Roger Mortimer-605 with a brother John, that wouldn't be prancing around in battles, in his 80s and 90s.
John seems to be a popular name, with these generations of Mortimers. Roger Mortimer-153 is the only Roger in generations missing a brother John.
There is a, or multiple, John de Mortimers running around in the late 13th and early 14th century, but I'm not seeing anything that definitively connects any of them, as a son of Roger Mortimer-81 and brother of Roger Mortimer-153.