Rig Veda Book 10, Hymn 21 is dedicated to Agni and mentions the following:
4 O Agni, what thou deemest wealth, Victorious and Immortal One! Bring thou to give us vigour,—at your glad carouse -splendid at sacrifices. Thou art waxing great.
5 Skilled in all lore is Agni, he whom erst Atharvan brought to life. He was Vivasvān's envoy, at your glad carouse-the weIl-loved friend of Yama, Thou art waxing great.
Are there any detailed stories about Agni & Surya or Yama in the scriptures that could justify these statements? I am not looking for their 'biological' relation but instances of their friendship or co-operation that can support the above statements.