WHAT DO YOU THINK?

I met my girlfriend through a her female friend and we've been dating for a year now often travelling to stay every other week at hers.

A while ago her sister visited from abroad for a short visit.  During her visit she met a guy but then gave my girlfriend phone number to this guy to contact my girlfriend before returning to her home country. This my sister in law even jokingly named this guy as her "sandwich  boyfriend" because he might had bought her some sandwich. To admit, my sister in law did not have any sexual relationship with the supposedly "sandwich  boyfriend" before returning to her home country according to my girlfriend. But sister in law suspected that he might had fancied her.

Astonishingly this guy has been calling my girlfriend and they've been chatting constantly. My girlfriend has told me that this guy wife has left him after I've tried to interrogate her. Recently,  my girlfriend told me that he's planning to visit her so they'll have some home made dinner together because they're both from the same ethnic background. He'll be driving some 70 miles distance.

I'm questioning the loyalty of my girlfriend. Plus why would her sister passed on her phone number to a stranger who is now coming to visit my girlfriend knowing  that she's already in a good relationship with me? I feel such visit would lead to reciprocal visits and a sexual relationship would then be established.

What would you think if you were in my position? Thank you in advance.