I honestly can't believe how self entitled they are behaving! I wonder if they would be happy to set your terms and conditions for attending their wedding.
Why does he want to wait outside? Does he think she might try to escape when he lets her out of prison for the wedding? I wonder if some reassurance that you won't let her escape or run away during the wedding would help?
I'm confused by this , my husband would happily take our children to the park and then meet us at the party venue. Knowing him he would actually prefer this. He would drop me off first, with a kiss and tell me to have a lovely time. But then he never was the jealous type, I would go out with my friends whilst he was offshore and he would call/ message me if he was on nightshift and ask if I had a good time. He would even give me his card and tell me to buy something nice to wear out.
As for wanting a role in the wedding, I think it is very clear that your sister is in a relationship with a first class narcissist.
www.huffingtonpost.com/dr-craig-malkin/narcissists_b_3331293.html
www.cheating-infidelity.com/Home/personal-stories-narcissistic-spouse/385-how-do-i-know-if-my-husband-is-a-narcissist-behavioral-signs.html
This one is especially relevant
www.psychologytoday.com/blog/communication-success/201409/10-signs-youre-in-relationship-narcissist