I found the solution. As said, there isnt any connection to the sale.order so I build a module to make this connection:

from openerp.osv import fields, osv

class account_invoice(osv.osv): 
    _inherit = "account.invoice"

    _columns = { 
         'sale_order_ids': fields.many2many('sale.order', 'sale_order_invoice_rel' 
         , 'invoice_id', 'order_id', 'Sale Orders', readonly=True, 
         help="The sale order(s) that generated this invoice. Multiple sales '\     
        'orders may have been combined for the same customer to generate one invoice."), 
	}

now its possible to nest the fields like this in the invoice pdf:

<span t-field="o.sale_order_ids.name"/>