Formal:
* Rwy'n caru fy nain. (Roo-in kar-oo muh nyn)
Informal:
* Dw i'n caru fy nain. (Dwi in kar-oo muh nyn)
Both mean "I love my grandmother" but the first is more formal and the second is more casual.
You can also use the affectionate term "Nain" instead of "Grandma" if you want to be even more familiar.
Example:
* Dw i'n caru Nain. (Dwi in kar-oo Nyn) - I love Grandma.
Let me know if you have any other questions!