Could it be Varun who is being referred to as Indra's brother here? I know the term Upendra is used for Vishnu commonly but the verse you have quoted actually just says Indra's brother:
According to the Aitreya Brahman many verses were sung one of which was to Indra and one to Varun but none to Vishnu.
He applied to Prajapati, who is the first of the gods, with the verse, liasya ndnam liatamasya. Prajapati answered him, 'Agni is the nearest of the gods, go to him." He then applied to Agni, with the verse, agner vayam prathamasya aviritdndm, Agni answered him, "Savitar rules over the creatures, go to him." He then applied to Savitar with the three verses beginning with, allii tvd devft Savitar. Savitar answered him, "Thou art bound for Varuna, the King, go to him." He applied to Varuna with the following thirty-one verses (124, 6-25, 21). Varuna then answered him, "Agni is the mouth of the gods, and the most compassionate of them. Praise him now then we shall release you." He then praised Agni with twenty-two verses (1, 26, 1, 27, 12). Agni then answered, " Prasise the Vishve Devas, then we shall release you." He then praised the Vishve Devas with the verse vamo mahaclbhyo namo arbhahebhyo. The Visve Devas answered, "Indra is the strongest, the most powerful, the most enduring, the most true of the gods, who knows best how to bring to an end anything. Praise him, then we shall release you." He then praised Indra with the hymn yach chid dlii satya somapd, and with fifteen verses of the following one (1, 30, 1-15). Indra, who had become pleased with his praise, presented him with a golden carriage. This present he accepted with the verse, isasvad indra. Indra then told him, "Praise the Asvins, then we shall release you." He then praised the Asvins, with the three verses which follow the abovementioned (1, 30, 17-19 . The As^vins then answered, "Praise Usas (Dawn), then we shall release you." He then praised UsAs with the three verses which follow the Asvin verses (1, 30, 20-22). As he repeated one verse after the other, the fetters (of Varuna) were falling off, and the belly of Harischandra became smaller. And, after he had done repeating the last verse, (all) the fetters were taken off, and Harischandra restored to health again.
There is a verse in the Rig Veda authored by Shunahshep (RV Book 1 Hymn 24) that quite fits the description too:
1 WHO now is he, what God among Immortals, of whose auspicious name we may bethink us?
Who shall to mighty Aditi restore us, that I may see my Father and my Mother?
2 Agni the God the first among the Immortals,—of his auspicious name let us bethink us.
He shall to mighty Aditi restore us, that I may see my Father and my Mother.
3 To thee, O Savitar, the Lord of precious things, who helpest us
Continually, for our share we come—
4 Wealth, highly lauded ere reproach hath fallen on it, which is laid,
Free from all hatred, in thy hands
5 Through thy protection may we come to even the height of affluence
Which Bhaga hath dealt out to us.
6 Ne’er have those birds that fly through air attained to thy high dominion or thy might or spirit;
Nor these the waters that flow on for ever, nor hills, abaters of the wind's wild fury.
7 Varuṇa, King, of hallowed might, sustaineth erect the Tree's stem in the baseless region.
Its rays, whose root is high above, stream downward. Deep may they sink within us, and be hidden.
8 King Varuṇa hath made a spacious pathway, a pathway for the Sun wherein to travel.
Where no way was he made him set his footstep, and warned afar whate’er afflicts the spirit.
9 A hundred balms are thine, O King, a thousand; deep and wide-reaching also be thy favours.
Far from us, far away drive thou Destruction. Put from us e’en the sin we have committed.
10 Whither by day depart the constellations that shine at night, set high in heaven above us?
Varuṇa's holy laws remain unweakened, and through the night the Moon moves on in splendor
11 I ask this of thee with my prayer adoring; thy worshipper craves this with his oblation.
Varuṇa, stay thou here and be not angry; steal not our life from us, O thou Wide-Ruler.
12 Nightly and daily this one thing they tell me, this too the thought of mine own heart repeateth.
May he to whom prayed fettered Śunaḥśepa, may he the Sovran Varuṇa release us.
13 Bound to three pillars captured Śunaḥśepa thus to the Āditya made his supplication.
Him may the Sovran Varuṇa deliver, wise, ne’er deceived, loosen the bonds that bind him.
14 With bending down, oblations, sacrifices, O Varuṇa, we deprecate thine anger:
Wise Asura, thou King of wide dominion, loosen the bonds of sins by us committed.
15 Loosen the bonds, O Varuṇa, that hold me, loosen the bonds above, between, and under.
So in thy holy law may we made sinless belong to Aditi, O thou Āditya.
Logically also a hymn to Varun would make more sense than that to Vishnu since the former is the one demanding the sacrifice!