Provider hosted app : throw error 403 on creating clientcontext to other site

I have created a provider hosted app, in which the app reads a list which is in other site . But I am getting Error 403 when I'm trying to create a ClientContext object of this site.

So if I have context of http://server/sites/site1 and I want to access a list in other site with url http://server/sites/site2. Is this operation possible in CSOM?

I already have the context of the site in which the app is running.

 ClientContext siteContext = new ClientContext("http://some_url");

Please let me know what am I missing.

UPDATE

Error message:

Access denied. You do not have permission to perform this action or access this resource