Hi Ian, welcome to TP.
Second tog (or reportage photographer) has a lot of le-way and not a lot of pressure.
DON'T get in the way of the main pro tog
DO take pics of the guests arriving......having a laugh.........informal moments
At wedding and meal......party after.........She will love you for it as the pro will have no time for this and Sis can see all Her Friends and relatives enjoying the Day.
For other tips,ask over on forum.
PS. STAY well away from the bar,as she is putting Her trust in you.