There are many possible solutions. The first that came to my mind is using VLC (video lan client) app like described here to access samba served files from your pc with what ever rights you want to give.
1. click on the files/folder with a right click and use share files to set up the share rights and f.e. a password.
2. download the vlc app on your sisters phone and tap on "local network" to access the samba folder share you shared on 1. (You both should be connected to the same network - boom - she can see the videos while you do stuff on your computer.
A second solution could be setting up an apache server with php support like described here or here for example.
Than create an index.php file containing this:
<?php
if ($handle = opendir('video/.')) {
while (false !== ($file = readdir($handle))) {
if ($file != "." && $file != "..") {
$thelist .= '<li><a href="video/'.$file.'">'.$file.'</a></li>';
}
}
closedir($handle);
}
?>
<h1>List of files in /Downloads/Video/:</h1>
<ul><?php echo $thelist; ?></ul>
This shows all files in a /video folder in /srv/http/ or where ever you got your homepage files in - maybe C:\dev\www\ .
Now you can take your sisters phone, type in the IP of your computer (f.e. 192.168.1.14 with opening cmd and type in ipconfig /all like described here) in the firefox app address line and it should show all your video files and you should be able to actually watch the video files directly by clicking on it - at least with the firefox mobile app.
She cannot delete any of the files if you restrict the access with samba (first method) and can't delete any files with the second method too. She can download files if she can read them - keep that in mind.
I hope that helped and answered your question :)