losing some one close is always painful. do your grandmother's good times a good time one last time, by reading a poem you wrote for her, thanking her for the time you spent together, et cetera.
do not ever worry about if it is up to her skill level, or if it will do her justice. it automatically will, because it just will. do not ask or second guess your abilities, for such a poem may open up the world for you in terms of writing. never ever (say never) be so disenchanted about yourself because of the situation. it sucks, yes. but it rocks too, like someone said: your grandmother is no longer in pain.
flip a coin: heads, write a poem; tails, write a different poem. either way, write a poem. since you stated that your grandmother was a poet, it will justify itself.