First of all, it is pointed out that couplets pay more attention to flat (flat), that is, the last word of the first couplet is flat (tonal rise and fall), and the last word of the second couplet should be flat (tonal rise and fall). Therefore, "show xxx longevity" is the bottom line ("autumn" is a flat voice); English xxx is the first part ("generation" is the sound). Maybe your grandmother's name is Xiuying, so I suggest changing the couplet to (meaning unchanged):
Xiumu lives longer than Songsong;
Huaying is like water.
(Remarks: Xiumu-a beautiful tree. The first couplet says: a beautiful tree is like a pine, but its longevity is not as old as Nanshan. Huaying-refers to the brilliance of the elite; It also extends to the moralization mentioned above. The bottom line says: goodness is like water, and happiness is like flowing water in the East China Sea.