The in-game browser approach is good for now. I finally solved to display the two camera image to separate eyes near real time.
I used two browser to display the separate images. I found this solution, how to display different images to each eye.
I think the new version of the unreal engine, and the separated html files fixed the previous latency problems.
In the future, I will try the engine built in media stream reader to display the video. But currently it can't read mjpeg_streamer.