Configure MongoDB Replication at AWS

I am trying to configure a MongoDB replication at AWS with 3 instances. I have two instances right now and I can ping each other. Mongo deamon is running correctly with a same replication name.

But after initiating the primary node, I cannot add the second instance withe a error

yaodao:PRIMARY> rs.add("yaodao2:27017")
{
"ok" : 0,
"errmsg" : "Quorum check failed because not enough voting nodes responded; required 2 but only the following 1 voting nodes responded: yaodao1:27017; the following nodes did not respond affirmatively: yaodao2:27017 failed with Connection refused",
"code" : 74
}

Someone sais the "coeection refused" is because no mongod deamon is running at second instance, but I verify that and everything is OK.

/etc/hosts for primary instance

127.0.0.1 yaodao1
54.173.155.9 yaodao2

/etc/hosts for second instance

127.0.0.1 yaodao2
54.175.238.78 yaodao1

rs.status() for primary

yaodao:PRIMARY> rs.status()
{
"set" : "yaodao",
"date" : ISODate("2016-09-17T13:34:01.032Z"),
"myState" : 1,
"term" : NumberLong(4),
"heartbeatIntervalMillis" : NumberLong(2000),
"members" : [
    {
        "_id" : 1,
        "name" : "yaodao1:27017",
        "health" : 1,
        "state" : 1,
        "stateStr" : "PRIMARY",
        "uptime" : 1132,
        "optime" : {
            "ts" : Timestamp(1474118471, 2),
            "t" : NumberLong(4)
        },
        "optimeDate" : ISODate("2016-09-17T13:21:11Z"),
        "electionTime" : Timestamp(1474118471, 1),
        "electionDate" : ISODate("2016-09-17T13:21:11Z"),
        "configVersion" : 59679,
        "self" : true
    }
],
"ok" : 1
}