Are patents WO2015191648A1 and US20190365145A1 valid? Another older patent covers claims 1 - 4. What to do in such a case?

First thing to point out, neither WO2015191648A1 nor US20190365145A1 are patents. Both are applications. Applications get reviewed by patent offices and only if they are deemed sufficiently new and novel issued as patents. Claims in applications are nearly always broader than the subsequent issued patent. I'm not a lawyer, but I'll take a crack at answering your specific questions.

According to WO2015191648A1, under legal events there is stated: "Ep: pct application non-entry in european phase". What does that mean? In my opinion that it is only published but not yet granted. Can it still be granted in Europe as it is already some years old?

Not exactly sure what this means. I'm assuming, but not sure that this means the application is still an application and not an issued patent. If it is ever granted, then it will be a patent, no matter when it was applied for.

in the US Patent version US20190365145A1 it has been publicated on 2019-DEC-05 (2019-12-05 Publication of US20190365145A1). As I understand this patent is not yet granted, only published?

This is only an application, not a patent. I try not to refer to applications as patents. An application may never be granted.

I found out, that especially the pump of the invention has already been patented in 1991 EP0496939A1. So I don't really see why the patent WO2015191648A1 can be done as the exactly same idea has already been patented many years ago.

Just because an element may be mentioned in a previous patent or other publication doesn't mean a patent can't be obtained. It just has to have some new and novel addition or improvement. Indeed patents can be granted for combinations of existing elements just so long as it isn't obvious to an expert in the field to combine them. I'd have to read and analyse the applications to tell if it has a novel element and I haven't done that. What I will say is the USPTO is usually pretty good at finding prior art within patent literature. By the way, the cited prior art is an application not a patent and it was withdrawn so it won't ever become a patent.

Is there a possibility to inform the patent-authorities about the fact, that claim 1 until 5 is already covered in EP0496939A1?

There are provisions to inform patent authorities about prior art. I doubt very much that they will fail to be aware of EP0496939A1 as it is in the patent literature. In any case, my quick review of EP0496939A1 would suggest it isn't enough to invalidate the cited applications. Let's look at claim one of US20190365145A1.

1. A device for creating infused beverages comprising:

   a solvent flow management system having:

      a pump body with a pump bore defining a pump interior volume;

      a piston disposed within the pump interior volume and in a fluidically sealed configuration with the pump bore; and

      a pump drive system operably coupled to the piston to generate a piston translation path;

   a solvent-flow conduit and in fluid communication with the pump interior volume and a brewing chamber; and

   an outlet for discharging an infused solution or receiving a flow of fluid;

   a flow control valve interposed between and fluidly coupled to the outlet and the brewing chamber, the flow control valve operably configured to have a selectively open position and a selectively closed position to control the flow of fluid through the outlet, the pump drive system operably configured to selectively drive the piston along the piston translation path, when the flow control valve is in the selectively closed position, to induce a negative pressure and the flow of fluid through a solute housed in the brewing chamber, downstream and through the solvent-flow conduit, and into the pump interior volume.

As you can see, there is a lot of text here. A claim doesn't protect every bit of content in it separately, it only protects all of the elements together. Thus if a claim has steps A, B, C, and D, then you only infringe on the claim if your device performs each and every step altogether.

Lastly, if you want to worry about an actual issued patent, you could look at US10349774B2 by the same inventor. It is an issued US patent.