Morning and glad you are here and asking.
If anyone has read my thoughts on this topic, the first thing I say is:
Don't Go.
I am completely serious. While I am an AA person and it was my first sponsor who told me "No is a complete sentence, you always have the choice to use it, and nothing is worth more than your sobriety." I pass that along in all situations (here, which is not an AA place, and in the group I lead for the restaurant industry which is also not an AA group, anywhere).
I chose not to participate or go to things for quite awhile. I know people, including you, will likely say but you have to, it's your sister, etc. You don't, though - it depends on what you are planning to do, and whether you will make the right (sober) decision so early in sobriety. Yes, so early. Four months is awesome - and very early - and you've shared that you don't really use any program, and you use a lot of words like afraid, etc.
However:
If you choose to go, as others have said, a plan is essential.
Here were my basics for the first party I attended (14 mo sober) and wedding (19 mo).
Have a sober pal (in my case my now husband, who is also sober - ie your other sister)
Let people (like that friend/sister) know you will not be drinking, regardless of how/why they know.
Keep a NA bev in hand or on the table next to the picture taking or in the dressing room or wherever. Literally, wherever you are. So what if no one else is or if you have to pee a lot?
Play the whole night thru - start to finish, whether that's getting your hair done, then to the church/whatever to get dressed, the ceremony, pics, so on to the next morning. What happened along the way?
Leave at the first sec you get that itchy feeling.
I think you get my point that it is a risky situation as you describe it, you don't have a lot of support (I would not consider popping into a meeting strong support tho not a bad idea at all) and it's up to you how you will wake up the next morning. I also feel very strongly that no one else's feelings or thoughts or desires matters when it comes to this - my money would be on my brother preferring I not get hammered and even by that alone emotionally destroy the special event for everyone, if that meant I didn't go.
As I said- it's up to you.
What's most important? For me, it was and is sobriety. At 3+ yr I deal with emotional sobriety around big events, and my capacity to handle them is much greater - but I am also completely accustomed to being everywhere sober.