Here are some thoughts. My mum used to make jam with a ratio of 1:1 1lb sugar, 1 lb fruit. Too long ago to remember, but it was beet sugar, rather than cane sugar, which is not as sweet. When I started my conserves company in Caracas, we made 5 different jams and I scaled down the ratio to 80% sugar, 100% fruit. That worked and passed all theMin. Health requirements for conserves. Next move was using fructose, which is about twice as sweet as sugar, so half the amount : 40% fructose, 100% fruit. I liked that, but it was very expensive to make.
Fast forward to 2014 - I had a visit from Tate & Lyle, who make sucralose ("Splenda" in its commercial format). Could I make some conserves with sucralose, they said. Sucralose, incidentally, is 600 times sweeter than sugar. Made a couple of chutneys and a couple of jams. Total disaster, because the
volume of the jam wasn't there and, to be honest, I had serious doubts about the preserving capacities of, for example, 1gm sucralose compared to 600 gms sugar.
The chutneys were passable, but sloppy, because the sugar cooks into a syrup, sucralose doesn't. The jam was awful - just fruit purée with sweetness, and almost liquid.
Tried with some of the other things (Aspartame, Stevia, Saccharin) and unfortunately, they leave a very distinctive, and often unpleasant aftertaste.
I think your best bet is fructose. Sure, it's more expensive than sugar, but since it's for family, not commercial use, you'll probably only need a few pounds of it.
