relative-guard-1.ts

@Injectable({ providedIn: 'root' })
export class MyGuard implements CanActivate {
  constructor(private router: Router, private route: ActivatedRoute) { }

  canActivate(route: ActivatedRouteSnapshot) {
    if(checkData(route)) {
      this.router.navigate(['../sibling'], { relativeTo: this.route }); 
      return false;
    }
    
    return true;
  }
}

relative-guard.ts

const routes: Routes = [{
  path: 'parent',
  component: ParentComponent,
  children: [
    {
      path: 'child',
      component: ChildComponent,
      canActivate: [MyGuard]
    },
    {
      path: 'sibling',
      component: SiblingComponent
    }
  ]
}]