I personally wouldn't give $$ to your stepmother, I would offer for any expenses that aren't covered. Every family is different, some family members may give his wife envelopes towards the expenses.
My mother passed a few years ago, I've never heard of the children paying for the tombstone. My father paid for everything, we weren't asked to cover anything. We purchased the flowers we wanted. But If you did want to help with the costs, I think paying for the tombstone would be a nice gesture from you & your brothers.
