If you ever need something shorter than a movie, Pui Pui Molcar on Netflix will be perfect. Maybe the French movie A Town Called Panic - which you do not need to know French for, it's stop motion with animal and cowboy little figurine toys and it's "acted" like the old silent movies so you get the basic story from the movements and plot without dialogue being strictly necessary. Miniscule is another maybe? There are some meanie flies and a ladybug is lost alone for a while, but the bugs are like bugs not people-bugs and there's no talking at all just bug sounds so maybe that would keep the intensity down?

Also, are your nieces okay? They seem a little old for some of the things you've said in your comments are scary to them. Do adults often watch with them and talk about the movies? Kids have to be taught how to process emotions and fictional events, they don't really get it through osmosis just being around older folks. If an adult is watching with them, children will often follow their lead in how they respond to strong imagery. Kind of like how toddlers won't always cry after a fall if the parent stays calm or says something like "wow! you're good at falling! can you show me how to jump?" An adult they trust and love sometimes commenting during or after intense scenes "wow, that would really bother me, I bet they figure it out though bc they are so good at problem solving" or things like that can really help. It helped my kids who did not like even mildly scary stuff, but take it or leave it bc I know there could be a situation I don't know or understand going on and your nieces are just being normal kids.