Why would this have been a secret till now?
Her father joined the Slovenian Communist Party, apparently for business reasons. Not sure if he still belongs, or if it matters any more, since he now lives in New York, but it would have caused problems for his business dealings back home, if his family were openly practicing Catholics. Given the liberal anti-Christian domination of the fashion industry, I'm sure that they won't approve, either. Thirdly, I suspect she may have feared it would cause problems for Trump's appeal to evangelicals, as well.
Maybe nobody asked?
“”Why would this have been a secret till now?””
I don’t think it’s been a secret. People just aren’t that familiar with the European countries which are predominately Catholic - IF they aren’t Muslim, that is. A lot of African countries are also Catholic which people don’t realize.