I hesitate to chastise you for automatically assuming that a First Communion invitation or notice (whichever it may be) is really a solicitation for money. So many people really do exploit the secular and religious milestones of their lives as opportunities to pass the hat that it is understandable that friends and relatives become wary of such attempts. Even so, it really would be in better taste for you to squash any such suspicions and treat this merely as a kind invitation you are unable to accept.
Just send a “short but sweet” email reply to your nephew thanking him for the invitation and letting him know you are unable to accept. (You needn’t tell him why. A simple “Thanks so much for the invitation. We wish we could be there but have another commitment that weekend” will do.) If you feel close enough to your nephew and grandniece to want to do a bit more, you could also congratulate your nephew on his daughter’s First Communion and let him know that you will be sending something to your grandniece. In that context, a card with a handwritten message of love and congratulations and a Bible bookmark tucked in would be charming.
And, if you have to do this with each family milestone that comes along, at least the only “cost” will be taking a few minutes of your time to express your love to your family on their happy occasion.
