Do people really have time to read the minutiae of daily life of numerous people from forums etc that they have never met? Can't the communication on the forum itself suffice? With those people, I don't see the point in keeping them as a friend but unfollowing - what is the point, then, in having them as a FB friend at all? I genuinely don't understand, not having a dig.
OP, there is no dilemma at all if there is only a tenuous link there. Just unfriend. There is nothing wrong with reading opposing political viewpoints on social media. when it is a part of an intelligent discussion. Some people clearly don't have FB Ranters on their newsfeed who come across as slightly deranged and get a bit of a Messiah complex when their all their mates appear to agree with them. The people who disagree with them never bother to reply, of course, because of the OTT response they get, but this perpetuates the problem and makes it even worse.
If these people were having a face to face discussion there would be immediate feedback, and body language to read. Social niceties would be present such as manners and turn-taking. But FB brings out the worst in people by just being a vehicle in which they can spout as much stuff as they want in a short space of time, with sensible people choosing to ignore, so the whole thing gets skewed. You can't ignore someone who is standing in front of you. If someone is annoying you with a political rant in real life you can change the subject or tell them to be quiet or let your body language do the talking.
The worst thing about it is you lose respect for people who you previously had every respect for when discussing things in person. When it gets to that stage (I'm thinking of you, BIL), you either need to unfollow, unfriend or do what I've done and delete the FB app off your phone . Going on the website takes more time and effort. I haven't looked at it today whereas I would just automatically click on the app several times a day to see if anyone has posted anything interesting. Feels quite liberating. The people I NEED to have proper contact with I can text, email or messenger, so no loss of communication.