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How would building these sites help you?.....Or would you be better off adding this information within the main site to build a better site
At this time I have already found it is losing visitors because they are not finding answers to their specific needs -- even when such answers can be found in the current main site.
Could they (niches websites) negatively affect the brand of main site?
Not if they are not duplicates. If they are as you say they will help each other
I would recommend using subdomains instead