Is there any problem to a girl in sucking the penis and swallowing the sperm in before marriage.?

No: No, but this is a scientific response not a religious one. Oral sex can transmit sexually transmitted diseases (stds) as well. This question is also very vague and difficult to answer. I can give you multiple scenarios and still may not answer what you are looking for.

Just the science: Of course this is a risk for sexually transmitted diseases, especially gonorrhea, both ways. There's some empirical evidence -- and it makes sense with what we know of the science -- that this reduces the risk for toxemia of pregnancy. It sounds like you have a relationship, and that you want to save the best for the wedding night. I admire you for this. Best wishes.