Thought I was the only one who noticed that- the girls moving their necks different from the boys! It also kind of looks like an accordion playing, right? Especially when they are excited or alarmed? Some young boys do this too but usually they don't do it so much once they are fully adult.
Blackshoulder hens can really vary a lot, some are much paler than deerman's picture, others are so dark they are more black than white. Many in between. Same thing in regards to the coloring on neck- some have none, most have a little patch high up on neck. Then there are spaldings, which are a mix of two species- Indian and Green.. spalding girls in black shoulder color can have very green necks. Also, spaldings can move their necks kinda funny so that's another possibility however your two girls look very typical India Blue in color.
Yes the father will breed the two once they are old enough. It will be OK. It's only bad if inbreeding goes on for several generations with no new blood added. A lot of people will object to this idea however inbreeding goes on far more often and widespread than most realize or want to admit.