@api.onchange method on a one2many field doesn't save the changes

I am trying to update sibling records in an @api.onchange method to "de-activate" them when another sibling is "activated". 

I am using the following code, but it doesn't update the relevant records:

class Child(models.Model):
    _name = 'module_name.child'
    parent_id = fields.Many2one('module_name.parent', string='My Parent')
    partner_id = fields.Many2one('res.partner', string='My Partner', domain=[('customer', '=', True)])
    is_active = fields.Boolean(string="Activated")
    @api.onchange('is_active')
    def _deactivate_siblings(self):
        if self.is_active:
            #the following line identifies siblings by parent id
            siblings_and_me = self.search([('parent_id', '=', self.parent_id.id)])
            #this line should update the siblings - but doesn't work
            siblings_and_me.write({'is_active': False})
            self.is_active = True

The code above does not "deactivate" the sibling records - So I end up with multiple active children. This should not happen.

I have also tried

self.search([('parent_id', '=', self.parent_id.id), ('id', '!=', self.id)]).write({{'is_active': False})

but this fails with a NewId error

Is there a different way to achieve this?

The parent Model is as follows:

class Parent(models.Model):
    _name = 'module_name.parent'
    child_ids = fields.One2many('module_name.child', 'parent_id', string="Children")
    active_child_partner = fields.Many2one('res.partner', compute='_get_active_child_partner')
    
    @api.depends('child_ids')
    def _get_active_child_partner(self):
        active_child = self.child_ids.filtered(lambda record: record.is_active == True)
        return active_child['partner_id'] if len(active_child) == 1 else active_child[0]['partner_id']

Furthermore,

The form view for parent does not display the active child's partner.

My Template is as follows:

<record model="ir.ui.view" id="parent_form_view">
      <field name="name">parent.form</field>
      <field name="model">module_name.parent</field>
      <field name="arch" type="xml">
        <form string="Parent">
          <sheet>
          ...
          <field name="active_child_partner" />
          <notebook>
            <page>
                <field name="child_ids" widget="one2many_list">
                     <tree string="Children" editable="bottom">
                          <field name="partner_id" options='{"always_reload": True}' domain=".." context="..."/>
                          <field name="other_field"/>
                          <field name="is_active"/>
                     </tree>
               </field>
          </page>
       </notebook>
    </sheet>
   </form>
  </field>
</record>

The field "active_child_partner" displays the label "Active child partner" but never has a partner populated there even though one child is active and has a partner.

How can I make sure that all siblings are updated (write should work in an onchange method)?

And how can I display the calculated partner?