I would like to be able to make my son's girlfriend dress modestly when she is in my home.
Why? Do you have a reason to require her to dress modestly?
He is such a modest and respectful young man, and I don't want him to get sexual attracted because of her dress
It's not okay to blame the way the girl dresses for your son not being able to control his sexual desires. Requiring her to dress in a modest way is unreasonable and will make her feel uncomfortable.
But she always visits in a miniskirt and bikinis. What if she is just trying to get my son horny?
Your son should be responsible for his own actions. How a girl dress in no way means he has to act on his feelings. If you confront her about her clothing choices you could make her feel embarrassed or ashamed.