i think you are making a big fuss over nothing - when they said "numbers" it could be that they didnt want to pay for you to come, after all weddings are expensive and if you have to invite every cousin, you also have to pay for a meal for them. Its an impossible situation to be in, deciding who is in and who is out especially on a limited budget. it might be that the groom had 57 cousins, so they made a rule "no cousins" dosent sound like you see them much anyway, so its hardly a loss to you.
as for the dying mum thing, how would it have looked to her that she was dying, its a time she wants to see you and you dont go with everyone else? You say you "genuinely couldn't go" If the woman means that much to you, what stopped you? you cant have it both ways. Did she die directly after the wedding? if not, i cant see how you could say it was your last chance to see her. Seems to me like you are being a big baby, and creating the bad feeling yourself