The word "relation" can mean a connection by blood, marriage or in other ways. 1. The study found a strong relationship (emotional bond or connection) between a lack of friends ..... 2. Whether you think the price is high is relative ( in connection with ) to the amount of ... 3. I am writing in relation (in regard or reference) to the job advertising .... Hope this helps

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For number 2, relative implies it can fluctuate. You could use related. It still makes sense. If I say, "It is relative," I mean depending on circumstances, it can change. If I say, "It is related," I mean there is a strong connection between the two without the nuisance of fluctuating factors. A relationship is just a noun and otherwise means the same thing as related. Just grammatically, it'll be used differently.