Looking for peach preserves substitute
Apricots (Prunus armeniaca and peaches (Prunus persica) are botanically speaking “close cousins” and have a quite similar flavor profile. Fresh apricot tends to be both sweeter and tarter, peach is a bit milder overall. The difference is less pronounced with preserves. 

For your sauce, apricot alone (i.e. using more apricot to get the same amount as the original apricot + peach) should be perfectly fine, assuming that you will adjust the sweet and sour other ingredients and seasonings to your liking anyway. 

If you find that apricot alone is too intense (unlikely, but of course there is no accounting for taste), I suggest some very mild fruit, e.g. pear or a sweet apple.