I'm in a similar boat with a sibling's wedding.
Sibling 1 married, spouse invited.
Sibling 2 not married, doesn't have relationships that last more than a couple of years (not judging, I don't care), partner invited.
Sibling 3, me, recently divorced (at time of original invites) partner not invited.
Originally when the invites were done B&G didn't know I was dating but after they found out and met I still wasn't allowed to bring DP to the evening. Fair enough, numbers were already done.
COVID hit, DP and I move in, serious relationship, sibling 2 splits with partner and a cousin pulls out due to no children rule. My DP still not extended an invitation. Then restricted numbers to 30, I wasn't included, as a sibling! I was on the bloody reserve list! Other siblings, plus the husband all included.
Wedding moved again to after 21st June, so likely no restrictions, DP only invited to evening despite spaces having opened up and that was only after I asked for clarification as all invite messages had no names at all on them.
I'm only attending the evening, with DP. The B&G showed me all along they don't give a toss about my feelings and are happy to treat me differently to our siblings so I'm happy (well I'm not happy, it feels like a slap in the face to me but I won't refuse altogether) with my decision to not go to the day alone.
Do what you feel is right OP as the sibling who is being treated differently (but I am biased).