Hi @motherofqilins
Well some people here seem to know a lot more information about your background than you give in your opening and subsequent posts, so I am quite confused.
From what I have been able to gather from everyone's posts on this thread, I think that your DM, and your Aunt, are sisters? After that, all I can make out, is that you seem to know - or think that you know - that your cousin in America and his Mum (your mum's sister) are as least as well off financially as you and your DH, and that they either do, or should, have the same priorities as you and your DH and DM do?
I think on one of your subsequent posts OP, you suggested that you went to quite a lot of expense in trying to help all who could, and/or wanted to, attend your Wedding; including paying for some peoples flights and accommodation? You now seem to be expecting your cousins family to do exactly the same?
So do you 100% know (direct from the horses mouth so to speak) that your cousin has as much disposable income and/or savings as you do? Do you also know, that if they have got lots of spare money, that they have the exact same priorities as you, and therefore, for example would be very happy to pay for both your DH and your DM to fly over to Italy, arrange and pay for accommodation for the 3 of you, and to have an extra guest at the Wedding Reception as well?
Without knowing any of the background at all in your opening post, I thought that the invitation addressed to "you and a guest", was deliberately worded that way, and that your cousin was trying to say, without having to spell it out, that he couldn't afford to invite both your DH and your DM, so he left that bit blank so that you could decide which one to invite yourself (not all people like sharing their financial situations with everyone, and may even find it rather crass to do so). Even if you believe that he (and his Fiancée) can and should spend their money, on their Wedding and Wedding guests, they might think differently.
For a start, at the moment in the UK we can still usually get adequate to good (and safe) schooling for our DCs, but I believe that at least in some parts of the US, it is very important to pay privately for your children's education. Also, for the time being - although goodness knows for how much longer - we have got included in our taxes etc, a National Health Service. Of course it isn't at all "free", but that seems to be how most prople view it. The US obviously doesn't have that, so the citizens have to pay for their Health Insurance - which may or not include any medication they might be prescribed - up front, and after any taxes etc have been taken from their wages. Many Americans will also worry about being made unemployed for any reason, unless they are multi-millionaires.
Then again, your cousin and his Fiancée might want to go an extended world tour for their honeymoon, and couldn't do that if they don't keep quite a tight reign on their Wedding costs. That wouldn't be them being horrible, nasty, or selfish, they don't owe any of you an amazing wedding experience at their Wedding. I am sure that for those who can attend, it will be both beautiful and emotional, and if I was you, and could go, I certainly would! I would personally take my DM, as she is his Aunty, and she might not have that many more weddings in the future that she will be able to attend.