aws elb describe-load-balancers --query "LoadBalancerDescriptions[*].{ID:LoadBalancerName,InstanceId:Instances[*].InstanceId}[*]. {ELB:ID,InstanceId:InstanceId[*]}" --output=json

[
{
    "ELB": "my_name",
    "InstanceId": [
        "instance-id-A",
        "instance-id-B",
    ]
},
{
    "ELB": "my_name2",
    "InstanceId": [
        "instance-id-B",
        "instance-id-C"
    ]
},
{
    "ELB": "my_name3",
    "InstanceId": [
        "instance-id-A",
        "instance-id-C"
    ]
}

aws elb describe-load-balancers --query "LoadBalancerDescriptions[*].{ID:LoadBalancerName,InstanceId:Instances[*].InstanceId}[?contains(to_string(@),'<InstanceId number')]"

[
    {
        "ID": "lb_name",
        "InstanceId": [
            "i-1234567890"
        ]
    }
]