...then how could they both have been direct descendants of the deceased John Jarndyce and possible beneficiaries of his Will ?
Given that primogeniture ( inheritance by the first born male ) was still in force in England in Dicken's time then as I understand it only Richard could have been a legitimate heir if he was a direct descendant. Ada as a female would have been ruled out.
IIRC Lady Dedlock also had an interest in Jarndyce vs Jarndyce. Females weren’t prohibited from inheriting and Primogeniture wasn’t legally required. (e.g. in Pride and Prejudice, Lady Catherine DeBourgh’s daughter will inherit Rosings Park and the remainder of her father’s estate.)
Entails were established to keep properties from being broken up by being split among siblings. But entails were not legally required either. The property/money in question in the J vsJ case wasn’t entailed.
Titles were inherited strictly by primogeniture, but without an entailment giving property to the eldest male heir or whatever the terms of the entailment were, property and money could be left to anyone in a will.
I'm not familiar with this story, and can't tell you what'd happen to an inheritance if someone died without a will or with a muddled will, you'd need an expert on Victoriana for that.