Well good luck on your move to NZ. I have always wanted to go there.

Now You know your mother best and it seems you are still talking to her, maybe some what limited. It also sounds like your sister is not on good terms. So she is controlling and sounds manipulating. First find out if you are invited to the wedding and reception. If this is a second for both they might not even be formal. Mine was Pioneer style in buck skins and some people even wore Shorts, Hawaiian Shirts and Teva. The Wedding and Reception was in small park and we set up a Lodge for part of the ceremony.

If you are invited, "wear clothes" as request on the invitation. If no mention of attire you and your husband can wear want you want to wear and have fun.
If you want to play the pipes bring them and let every one see them and I am sure you will be asked to play. I bring my Harmonicas every where and am asked to play alot.
Nothing wrong with being a little pushy.

The main thing is if you go have fun if no fun is happening you can always leave. After all you are going to NZ.

Lots of luck,
MrBill