My SIL has been in our family for about 10 years now and I have met her brother plus partner several times over the years.
I really like them both and think they are lovely people and a great couple.
I am not going to the wedding and would never have expected to be invited, I like them but it wouldn't even be annually I see them. But I have seen them 3 times in the last year.
Anyway, I know I can often be a bit 'odd'
Can I send them a wedding card + voucher?
YABU - Don't do it
YANBU - Send a card