V.C.
I know this is a hard one however if it's possible She should go to the reception for a little while and then have a sitter (whomever would watch her after the wedding) pick her up. Maybe the SIL won't mind if she goes straight home, I woul ask to get a feel for it, just simply explain it's way too long of a day for her, she can't be expected to make it the whole day.
For my flower girls that's what we did, someone picked them up and took them home, however I did not have the big gap in between for that reason(I too had 2 flower girls).
Good Luck