[Sinister-Kisses]

Video would help. Is she actually SWIMMING sideways, or is she just hanging out sideways? If there are not enough hiding places in the tank for her to feel comfortable, they will often hang out sideways/upside down/whatever odd ball angle trying to "hide" wherever they can - corners, behind or next to filter intakes or heaters, etc. If that is what she's doing, the concern would be having adequate hiding spaces for your fish. If she's actually swimming sideways, that's a whole different issue.

Side note - a group of only 3 demasoni will not likely do well long term.