An heir is someone that inherits from an estate due to being a descendant or relative of the deceased.
Yes, "heirness" is considered a neuter noun as it does not have a specific gender association. It refers to an individual who inherits, regardless of their gender, although in modern usage, "heir" is more commonly used and encompasses all genders. However, "heirness" is not widely recognized or used in contemporary language.
bull-cow cock-hen gentleman-lady baron-baroness heir-heirness host-hostess lion-lioness fox-vixen landlord-landlady shepherd-shepherdess brave-squaw duck-drake emperor-empress count-countess mademoiselle-monsieur tiger-tigress whale-bull peacock-peahen (GOAT) nanny-goat-billy-goat aunt-uncle boy-girl abbot-abbey man-woman lad-lass tailor-dressmaker/seamstress bachelor-spinster duke-duchess prince-princess actor-actress king-queen widower-widow rabbit-doe deer-doe master-mistress sir-madam businessman-businesswoman father-mother grandnephew-grandniece nephew-nice brother-sister lord-lady grandfather-grandmother hero-heroine bride-groom groomsman-bridesmaid sister-in-law-brother-in-law father-in-law-mother-in-law milkman-milkmaid governor-matron/governess prophet-prophetess poet-poetess waiter-waitress papa-mama kuya-ate dad-mum daddy-mummy dog-bitch alumnus-alumnae alumna-alumni steward-stewardess son-daughter wizard-witch husband-wife lolo-lola priest-nun