SYRUP: Combine sugar, water, rhubarb and strawberries in a medium saucepan. Boil over medium-high until the fruits turn mushy when pressed, 8 to 9 min. Strain, using the back of a spoon to press liquid out of fruit. Cool completely. Will keep, refrigerated, up to 1 week.
ONCE syrup is strained, add lemon juice and stir in gelatin until dissolved. (It will foam.) Stir in wine and club soda. Add a few sliced strawberries to glasses or ramekins, then top with sangria jelly mixture. Refrigerate until softly set, about 6 hr (preferably overnight).
Chatelaine Quenchers: How to make lemonade slushies