Before we can move forward, you have to make up your mind and decide if ethnicity and nation are a cultural-linguistic concept (which is the idea that got you started), or if it is something else (now you've just derived to "bio-cultural", which you should explain).
I can't get your point, whichever it may be, when your example is not only not the best but in reality it is the worse that I've seen in a long time. I mean, you haven't made any point with it, other than the point of being wrong.. sorry to say.
As for the rest, Galicians are as related to the Central and Southern Portuguese as the Central or Southern Portuguese are related to Murcians. As for the relation of Galicians with the Northern Portuguese, this has diminished and will surely diminish more in the future while the Portuguese state denies them their identity and they eventually lose it.
p.s. actually, the relation of Central Portuguese with Murcians could be biologically closer, if the Alans left any print in both since they settled for a time in both ends of the Peninsula.
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'Dardanidae duri, quae uos a stirpe parentum
prima tulit tellus, eadem uos ubere laeto
accipiet reduces. Antiquam exquirite matrem:
hic domus Aeneae cunctis dominabitur oris,
et nati natorum, et qui nascentur ab illis.'
We can easily forgive a child who is afraid of the dark; the real tragedy of life is when men are afraid of the light.
–Plato–
'Many people, I believe, wish for a society where faith, decency, pro-life convictions and national self-determination within Europe can flourish; and not be swallowed up in a dictatorial EU bureaucracy.'
–Gerry McGeough, Irish Nationalist and POW–