Ajaxly login/signup in bootstrap modal
There are two questions:

1.) goal: open login form in bootsrap modal.

In your code by redirecting to a new path the page will reload, and not display a modal login.

But you don't need a new path, your form is already there. It is in the block content and you should be able to place {{ content }} in the bootstrap modal dialog like this.

block--userlogin.html.twig:

&lt;div id="signin" class="modal fade" role="dialog"&gt;
    &lt;div class="modal-dialog"&gt;

        &lt;!-- Modal content--&gt;
        &lt;div class="modal-content"&gt;
            &lt;div class="modal-header"&gt;
                &lt;button type="button" class="close" data-dismiss="modal"&gt;&amp;times;&lt;/button&gt;
                &lt;h4 class="modal-title"&gt;Modal Header&lt;/h4&gt;
            &lt;/div&gt;
            &lt;div class="modal-body"&gt;
                {{ content }}
            &lt;/div&gt;
            &lt;div class="modal-footer"&gt;
                &lt;button type="button" class="btn btn-default" data-dismiss="modal"&gt;Close&lt;/button&gt;
            &lt;/div&gt;
        &lt;/div&gt;

    &lt;/div&gt;
&lt;/div&gt;


2.) ajaxifing the login (not reloading after submit)

For this there is some work to do, you can see this in the D7 module:

https://www.drupal.org/project/ajax_register

For D8 you need to code something similar.