Node js vs Django vs Flask for multiple videos streaming from python
I would say it doesn't really matter much as Django would be good because of its  pre built features... Plus data transfer is very reliable in django along with your expertise in python, django is preferred. NodeJs would be easier to implement tho. Really prefers upon you, but both django and nodeJs works pretty well.
Sorry, no idea on flask
I got 6 real-time videos which are inference output from heavy deep learning calculation from python, and I've tried to display them to PyQt that lots of threading issues with ugly GUI!!. So, I want to use a framework only to display well and fancy GUI! There seem lots of frameworks based on my google search, and I have no idea 
which one is the best for my current project among Node js, Django, and Flask!

I need to display 6 real-time videos with 10~15 FPS.
Communicating well with python.
Easy to build GUI, I have some features like logging, displaying real-time graph(optional)