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Yes adelphopoiesis could be used to join multiple persons as siblings. It was also used for children who were ether orphaned or needed to be sent abroad due to war or economic calamtities (or whatever). They would have a blessing to make them siblings and have each others backs.
The most famous case is that of Photius Kasavales Fisk, who was a U.S. Navy chaplain in the 1870's. At 15 years old he was orphaned from the Greco-Turkish War of 1821, and a Protestant missionary (Pliny Fisk) found him living amongst relatives in Malta. He secured Photius an education in America, where the local Greek priest, after examination of Pliny Fisk's intent, also sent his 11 year old son Anastasius together with Photius for a better life. It was agreed that the boys would not be separated upon arrival in America, and the priest performed the rite uniting the boys as brothers.
You can read about it from Photius Fisk biography, written in 1891. Note this blessing has nothing to do with marriage or sexual unions (that's an invention of the last few decades). It is more akin to an adoption ceremony. Read Photius Fisk: A Biography, by Lyman F. Hodge pages 23-26.