[{_id:"1",city:"NY",user:[{firstName:"John",lastname:"Peters",brothers:[{_id:"B1",brotherFirstName:"Karl"}]},{firstName:"Raul",lastname:"Other",brothers:[{_id:"B2",brotherFirstName:"May"}]},{firstName:"Paul",lastname:"Ray",brothers:[{_id:"B3",brotherFirstName:"Bryan"}]}]},{_id:"2",city:"NY",user:[{firstName:"Joe",lastname:"Anders",brothers:[{_id:"B4",brotherFirstName:"Carla"}]},{firstName:"Zoy",lastname:"Bat",brothers:[{_id:"B5",brotherFirstName:"Stuart"}]},{firstName:"Ana",lastname:"Lily",brothers:[{_id:"B6",brotherFirstName:"Carter"}]}]}]

db.collection.aggregate([{"$unwind":"$user"},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$$ROOT","$user"]}}},{"$unwind":"$brothers"},{"$replaceRoot":{"newRoot":{"$mergeObjects":["$brothers","$$ROOT"]}}},{$project:{brothers:0,user:0}}])