I was kind of hoping to see how people interpreted the situation rather than being told “just ask” over and over again! If this was you, how would you view it?
I would view it as a confusing and vague invitation that therefore needed clarifying --and wonder why on earth the hosts didn't put "MiL. FiL, BiL, SiL, OP'sDF and OP" or "MiL & FiL" or whatever combination of people they actually want to invite.
If my in laws received an invite to ‘the Lawnfevers’ to their home it wouldn’t occur to me that it would be an invite for DH, me or his other siblings who don’t live at home.
Though, just before COVID, we did get an invitation to our house, addressed to "The Kites" , and, when we checked, they did want to invite our adult dc who were all a bit transient at the time, and the couple getting married didn't know their addresses...... so it just goes to show that people are different, and I am glad we checked, as were the B&G, and as were our adult dc.
I know asking is the best option but if my cousins fiancé asked me if they were invited to my wedding, I’d be baffled!
I don't understand why you would be baffled. If you send out vague, unclear invitations, then you would be bound to get queries. Though I would expect it to come from my cousin, not his fiance.
As others have said, this is perfect Hi Cousin,
Mum and dad have said we're invited to your wedding but I think the invitation was just for them, can you clarify? (but your fiance needs to send it really, not you of course).