What do you think about me saying that my sister will always be her maiden name at her wedding.
Why do you need to bring this up?
I think her boyfriend should be more responsible towards this matter. He didn't take his name when he got married before.
People getting married have the choice to take a new last name or not.  You should respect your sister and her husband's choices, and not try to cause a disruption at their wedding.
I just hope she takes the right last name after she's married, is all.
If your sister gets married then that is her choice, as well as who's surname she takes after marriage. You should respect her choices, or people will think you are pushy and rude.