Olympus E-PL7 or Sony A5100?

How important is that?
You already have at least one m4/3rds lens on your e-pL1, right? Maybe that one is better for selfies, which would save the day.

 

I think there is no practical difference and are both great cameras. Quality is close, and I would expect the Sony kit lens (24-75 equiv 3.5/5.6) to be on par with the Oly one (28-84 equiv 3.5/5.6). I'm not a selfie expert   but I do not think that the 24-28 difference is going to matter much, maybe for group shots.

If you plan to buy other lenses things change: Oly lenses are smaller and there are a lot more (Sony will add more in the next years) and usually are a little cheaper.

The biggest sensor of the Sony should allow you to crop a little more aggressively. The EPL-7 should have a couple of dedicated "selfie modes" like "touch the screen to take a shot with a small delay or a sequence of shots".

If you do not intend to buy more lenses the Panasonic LX-100 could be a good alternative (better lens than both) but it costs more.

Is there anything that you felt was not good enough in you E-PL1 that could be an hint to choose the next one?

 

I cannot comment on the Sony, but have been using the E-PL7 pretty much since it came out. I shoot for the same reasons you do, and might be on the same level. I am more than happy with the E-PL7: I find it small enough to be transported easily, I did the image quality and am happy with the available lenses (especially when on a budget). As you are already used to the Oly, and since the E-PL7 is a big step up from your PL1, I don't think you can go wrong with it.