Just please try addin the same data rel attribute to the video, it’s not very consistent but it might work.
To be precise, the best syntax for galleries is:
<a href="img url" data-rel="lightbox-gallery-1">thumbnail</a>
<a href="img url" data-rel="lightbox-gallery-1">thumbnail</a>
<a href="video url" data-rel="lightbox-gallery-1">thumbnail</a>
And if you have another gallery just set the data-rel to lightbox-gallery-2, 3, 4 and so on.
RLG is doing the same internally, but for custom galleries this is the way to go.