I think it's time you put yourself first, for a change!!
You had the finances before therefore you were happy to give gifts to that side of the family. But things have changed and you no longer have the money to give. Surely, as you say you are a pensioner now, these nieces & nephews will understand, I mean they must be old enough to realise that money does not grow on trees!!
Think of yourself, first & foremost, then of your own immediate family (your child/ren), then welcome the new grandchild with your loving arms, happy that you can occasionally spoil him or her!!!
Remember, money does not grow on trees, and to put yourself first!!!!