What's your opinion on marriage/serious dating & pro athletes?

My sister has been dating a pro baseball player for almost 4 years... She travels and stays with him during the season... He seems to really care about her but I'm beginning to wonder if she is wasting her time... I can't really tell if his feelings for her are sincere, and I am starting to believe that he just wants someone around so he won't be coming home to a lonely house after a tough game... he's a nice guy and they get along...and, i know that marriage is important to her and I also know that it doesn't take 4 years to decide if you want to marry the person...and they are both definitely at a marrying age... this is why I think she may be wasting her time with him. bc i know i would not stick around for that long if marriage was important to me. On top of that, he is a good looking pro athlete so him being completely loyal to her is very unlikely I think...and i want whoever she marries to be devoted and good to her...i haven't talked to her about this bc i dont want to hurt her feelings...the divorce rate for pro athletes is 60-80%...infidelity being the main cause...so what do you guys think?