Many plants can be grafted on plants of another genus, within the same botanical family.
Poshte' or Cawesh, Annona scleroderma, is a true Annona (Attae Group), with narrow-pointed flower buds, with 3 petals (sometimes with minuscule three vestigial petals between the noticeable petals), and sweet-smelling flower aroma. These species tend to be interbreedable, within the "group."
Soursop and Mountain-Soursop, and many other species, are in the "old" Guanabanus genus, which most botanists prefer to call Annona (Eu-Annona Group). These have huge heavy flowers with 6 wide petals, enormous pollen grains, and a chloroformic / alcoholic stench.
These two separate genera, or separate Groups, do not share insect pollinators, and humans have not succeded in getting viable seeds from cross-pollinating species in these different groups. So they are NOT "closely related," but may well be graftable anyway.
If similarity of leaves determined inter-graftability, you could graft soursop onto many Ficus species. Not so.