She was dressed accordingly. Just not to your standards. There was nothing immodest, or unseeming about her outfit. She wasn't in ripped clothes, showing skin or anything else wrong. She made the Walk to church, which is not a short jaunt across the road. If she chose to wear more comfortable trainers to make the earlier walk, good on her.
I see no reason to mention how often you attend. Its no competition who is the better church goer.
As for Sarah or anyone else being there or not....they find a great balance. The only new editions I'd think possible when Charles is king, would be his stepchildren and their families. I don't see him allowing Sarah to join, even after his father passes, unless Andrew remarries. It would be nice if Camilla's kids were included, but I don't think they will. The division will be maintained likely, and Camilla will likely continue as she does. Or perhaps they will stay at Wood farm, and be part of the private celebrations but not the church walk.