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I work in a small (~25 FTE) ICT company related to energy infrastructure in Germany, Europe. I've recently been promoted to Department leader due to the people above me moving on, and thus have started getting involved in the hiring process.

Early this year, we hired 3 new people. One of them displayed very poor workplace morale (constantly on the phone, showing no initiative on their own...) as well as a severe lack of skills:

They had supposedly finished a course in Electrical Power Engineering, as well as published a research paper on State Estimation in electrical grids. They didn't know Ohm's law, and when asked "What's a Transformer?" they started talking about AC/DC conversion.
According to the documents, they studied at the same university as I had, in roughly the same time frame, taking most of the same courses. I can personally attest that the professors there do not hand out grades for free.
So, after about a month of experiencing this a the new boss, we let them go, suspecting a case of fraud but deciding not to pursue legal steps.

Sidenote: I even checked and compared their signature towards what was on their drivers license - turns out they signed in block letters, literally the only person I ever saw doing that, and making forging a signature easy.

Another case came in recently for an interview. They had programming in C/C++, Pascal and Python on their resumé, did a bachelor in telecommunication and ICT, and were currently working on their master's thesis on a battery management system for electric vehicles, some 8 weeks into the thesis.
They couldn't explain what a Battery Management System actually does, and when asked 'Which compiler/development environment are you using?' they didn't seem to know what I was talking about, i.e. what a compiler even is. They also didn't display any other skill in the fields they supposedly studied in.

A third case, this time for an internship, went really well and they knew their stuff brilliantly, especially for someone who's just applying for internship. When we offered them the position, they declined, saying they found something elsewhere, 'But my twin sibling is looking for an internship in the same time frame'. We invited the sibling to send over their CV, we'll see how they do.

All of the three cases above have some similarities in their demographic traits, including a migrant background from roughly the same cultural area.
We checked all of the CVs and degrees as well as we could, but did not see them to be fake. We called former employers and found that the story matched what we read in the CV.

So, the questions would be:
Is there some kind of fraud scheme going on where people take their siblings accolades to apply for jobs?
Perhaps an uptake in forged degrees in Europe, possibly related to the migrant influx?
Is it legal for me to call up the university and ask about a degree that I suspect to be fraudulent?

P.S. I'm pointing out the migrant background because it might be relevant to the answer. Out of the 3 people we hired initially, all of them had a migrant background, and the other 2 worked out beautifully and will get perpetually employed.