Here's the background: DH has a cousin (we'll call him Bill) who was estranged from the family for about a decade. After reconnecting with his father last year, Bill and his fiancee threw a birthday party for their daughter, and opened the invitation up to the entire extended family. We did not go for a few reasons - it was out of town, during DD's nap, she wouldn't sleep in the car, etc.
Anyway, Bill got married last weekend, and long story short, wedding invitations were only sent to the people who came to the birthday party. An Aunt who is in bad health was invited to the wedding, but could not attend. Instead, she sent her son (DH's and Bill's cousin), who was NOT invited to the wedding, in her place.
Is this proper etiquette? And, etiquette aside, how would you feel in this situation?
MIL and I got into a discussion about this, and she insisted to me that it was perfectly acceptable for the aunt to send someone else in her place. I argued that an invitation is for the person to whom it is addressed, and that person alone. I think I would have been really uncomfortable if uninvited people showed up to our wedding in someone else's place, family or not!

Re: Wedding Etiquette Question
If this comes up again, I'm going to re-question my MIL. I'm hoping I misheard or misunderstood what she was saying. She is usually schooling me on etiquette and what is proper, I am so stumped that she would think this is ok!
Luckily, DH doesn't have any hard feelings, it's not like we even know them. As for myself, I was relieved that we didn't have to find a babysitter and buy a gift
If it is not a close connection (like a in your case) then no. I would just mail the gift and regrets.
Maybe the Aunt did it to show suppprt for a recently reunited family member? And also, maybe "Bill" thought the ones who didnt come to the birthday werent very interested in him so didnt want to push another invitation on them?