Bismihi ta'ala

It's greatly noble that they are very good people, and for sure their nurturing contributed to your personality as well. 

As you know, religion and becoming religious is a choice that must come from within. Even as Muslims, we always pray to Almighty God to keep us "guided" on the right path. Hidayah, or guidance is in the hands of the Almighty.  If your grandparents, or anyone else around you has a potential that Allah ta'ala sees in them, then they will be guided to the light of Iman. If they do not "convert" to Islam, it does not mean they are evil, but just that hidayah is with the Almighty. 

The best thing you can do is set yourself with high standards in your conduct, in your ambitions in life, and show those around you how a true Muslim should be. 

That itself is a great form of conveying the message of Islam, and then it is up to them to investigate, or ask questions, and so on. 

If you see that your grandparents are interested, or have questions, discuss what you know with them. However, never allow it to become an argument, or upset them, as they are your grandparents. 

In any case, always do dua for yourself, your parents, your grandparents, your siblings and everyone else, that God showers them with His compassion and Mercy. Ask the Almighty to enlighten your hearts, and ask Him for guidance. 

You can also do tawassul to Ahlul Bayt (a.s.), to give you the strength you need, and introduce them to Ahlul Bayt (a.s.), in a positive manner. You dont know what the outcome will be, but at least they will think positive of your religion, and understand you more. 

With prayers for your success.