Hi itchybackside
All dates will definitely clash to someone. Your sister can be the jie Mei or helping you in the wedding as key personnel that is all. And she have to hide during gatecrash till after groom step in then can come out
But the date cannot clash to parents. If so, you cannot use this date
So 21/6 is out
26/7 clashes to dragon, cannot be used
2/8 is in 7th mth
6/9 clashes to dog, none of you and is a general good date
13/9 clashes to snake, so cannot be used
20/9 clashes to rat which is none of you, but not a general tong Shi good date
Hope this helps