Your sister is about 34-35 years old and you have not responsibility for her. She is old and it´s her life.
I hope he is a good man but I´m not sure.
I also know there are many bad men and ladies who are member of Thaicupid. and many thai girls get broken heart and babies from foreign men.
I understand you thinking about your sister much. Is the guy from Denmark her first boyfriend from another vountry or from Thailand?
Take care of you herself and have a nice day!
Hugs