How do I output the siblings position number?
Building upon the answer I posted to this question How to get the order number of the current entry in a structure, you should be able to get the order number in your for-loop like so:

{% set parent = entry.getParent() %}
{% set childrenIds = parent.getChildren().ids() %}

{% for sibling in entry.getSiblings() %}
    {% set orderNumber = craft.entries({
        id: siblingsIds,
        order: 'lft desc',
        positionedBefore: sibling,
    }).total() + 1 %}

    {{ sibling.title }} ({{ orderNumber }})
{% endfor %}