I do feel sorry for you OP, and your sister. This would take all of my skills to try to manage it to get what I wanted without him using it later on as an excuse to drive a wedge between me and my DS. Can you talk to her on the phone so that he isn't manipulating the messages?
I don't think sitting outside the room is a good idea. He might barge in, or be let in by some kind person who doesn't know better, and if he is waiting outside you might feel tense about what might happen after the ceremony during that special moment that is utterly yours.
There is also potential for embarrassment if someone says the wrong thing afterwards. It happened to me eons ago at the wedding of a friend. One night I got off with a good friend of mine - it was a delicious night and there were no complications afterwards, ever, we went straight back to being friends. Fast forward three years, I was in a long term relationship with a mutual friend and he was getting married to a woman he'd recently met who'd got pregnant straight away. He asked my boyfriend and I to come to the reception but he asked me if I wouldn't come to the church. Unfortunately the church was in the middle of nowhere so I ended up having to drive up with OH and wait outside the church. When the ceremony ended I just walked from the churchyard to the reception with everyone else, hoping I wasn't noticed and was horrified for the groom when the bride asked me why I wasn't in the service. It was an entirely innocent thing that just turned into something awkward and I wished I'd handled it differently. His being outside will just remind everyone that you were the person who didn't let people in!
And about the roll, I'd say yes would be delighted to accommodate to this particular request, which would he prefer, cheese or tuna?
Anyway, good luck to whatever you decide to do and have a beautiful day.