import cybox.objects.file_object
import stix.core

stix_package = stix.core.STIXPackage()
file_object1 = cybox.objects.file_object.File()
file_object2 = cybox.objects.file_object.File()
stix_package.add_observable(file_object1)
file_object1.add_related(file_object2, 'Related_To', inline=False)
stix_package.add_observable(file_object2)

print(stix_package.to_xml().decode('utf-8'))

<stix:STIX_Package 
        xmlns:xs="http://www.w3.org/2001/XMLSchema"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:FileObj="http://cybox.mitre.org/objects#FileObject-2"
        xmlns:cyboxVocabs="http://cybox.mitre.org/default_vocabularies-2"
        xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xmlns:cybox="http://cybox.mitre.org/cybox-2"
        xmlns:example="http://example.com"
        xmlns:stix="http://stix.mitre.org/stix-1"
         id="example:Package-0981f992-21e7-4482-8b77-81c3ed00c94e" version="1.2">
    <stix:Observables cybox_major_version="2" cybox_minor_version="1" cybox_update_version="0">
        <cybox:Observable id="example:Observable-7802bae4-958f-418e-b146-093761399aaf">
            <cybox:Object id="example:File-b58b6afe-e586-449e-83e9-63ebf0a4cce8">
                <cybox:Properties xsi:type="FileObj:FileObjectType">
                </cybox:Properties>
                <cybox:Related_Objects>
                    <cybox:Related_Object idref="example:File-1a2b459a-9b2f-4ef6-9fad-a78784b00744">
                        <cybox:Properties xsi:type="FileObj:FileObjectType">
                        </cybox:Properties>
                        <cybox:Relationship xsi:type="cyboxVocabs:ObjectRelationshipVocab-1.1">Related_To</cybox:Relationship>
                    </cybox:Related_Object>
                </cybox:Related_Objects>
            </cybox:Object>
        </cybox:Observable>
        <cybox:Observable id="example:Observable-34c3c3f4-185a-4ce1-be5e-b2357f32115d">
            <cybox:Object id="example:File-1a2b459a-9b2f-4ef6-9fad-a78784b00744">
                <cybox:Properties xsi:type="FileObj:FileObjectType">
                </cybox:Properties>
            </cybox:Object>
        </cybox:Observable>
    </stix:Observables>
</stix:STIX_Package>