Cherimoyas are derived from a tropical tree, of the genus Annona. They are not related to the jackfruit. Although both can be found in India, only the jackfruit is a native of India, while cherimoyas are from South America. Actually, the jackfruit is related to the breadfruit, and fig, while the soursop and custard apple are related to the cherimoya.
It's true that their taste may be similar, but this would be due to the fact that both are high in starch, and sweeten as they ripen, each with strong aromas, nevertheless, they are not related.
Hope that helps,
Peter Bosani.
P.S. For more info on food connections, you can check out my web site at: www.myspace.com/bosani