Sirius no Densetsu (Sea Prince & The Fire Child) is a 1981 anime movie produced by Sanrio and one of the highest achievements of japanese animation, clearly paying hommage, in the most tasteful way, to Disney's Fantasia (especially the "Nutcracker suite" and the "Night On Bald Mountain" segments).
Bluray :
The 2013 remastered Bluray is presented in fullscreen 16:9, cropped vertically.
In this screenshot, Malta (the fire child) has the light on her head missing due to the vertical trimming :
DVD :
The 2010 american DVD is in 4:3 which clearly seems to be closer to the original format.
Looking at the same frame, here we can actually see the little light on her head.
Full frame :
The original japanese trailer presented on the 2010 american DVD shows an even slightly bigger frame than the DVD image.
Looking at this particular image from the trailer, if we add the DVD and the bluray we have a global aspect ratio of 1.39:1 which is probably close to the original format.
Conclusion :
Sirius no Densetsu is not a widescreen movie, its original aspect ratio is probably close to 1.39:1.