1) Estate sharer 2) Estate splitter 3) Joint beneficiary 4) Joint inheritor 5) Joint legatee 6) Legacy sharer 7) One sharing the wealth 8) One who shares in a will 9) One who shares the wealth 10) Relative of a legatee 11) Sharer in an estate 12) Sibling, often 
• (n.) A joint heir; one of two or more heirs; one of several entitled to an inheritance.
Co·heir noun A joint heir; one of two or more heirs; one of several entitled to an inheritance. 
a joint heir.