My SO's sister is getting married this summer and I really

Fake a depressive episode

You can't just suck it up for a few hours, even for the sake of your SO?

SO isn't specific enough to establish whether or not you're being a bitch about this.

If it was your SPOUSES sibling, then searching for an excuse to get out of it just means that you're a piece of shit.
A fiancé, it's kind of the same thing. If you care about this person enough to commit to marry them, then it is stupid and futile to try and get out of a wedding, because as a member of the family that shit is going to keep coming.
Other than that, there are a million ways to get out of it, but who are you convincing? Your SO, or the sister?? for the sister all that's required is food poisoning, or finals, or a ful earl or job interview. But if it's your SO. You have to convince, then you have to decide between truth and a lie, don't you?