It depends to some extent where in Rawmarsh you're starting from, but here's one 16 mile option:
https://cycle.travel/map/journey/280510
Generally quiet lanes/roads couple with a good proportion of firm cinder/crushed limestone trail, although there might be a few puddles given recent weather. It's lovely in summer. Wildlife (Old Moor wetlands), heritage (Elsecar Branch Canal & the Heritage Centre), and history (Holy Trinity Church in Wentworth and
Wentworth Woodhouse definitely worth a look ... although these may of course be familiar to your SiL).
Several cafe options: RSPB Old Moor Visitor Centre, Elsecar Heritage Centre, Village Tea Rooms or Garden Centre in Wentworth.
It's a bit of a climb from Elsecar up to Wentworth (and there's a short one approaching Wath), but otherwise OK, especially if SiL is on an e-bike.