For that you can use fields.related in sale.order.

Try this:

' partner_family_name': fields.related('partner_id', ' family_name', type='char', string='Partner Family Name', readonly=True, store=True),

fields.related is a field that points to some data inside another field of the current record.

Example:

_columns = { 'partner_id': fields.many2one('res.partner', 'Partner'), 'partner_name': fields.related('partner_id', 'name', type='char', string='Partner Name'), }

Here Related Field you can find more about fields.related.