I was going to say something along the same lines ... or rather, that from the book we learn that the Bennets gave up hoping for a son some years after Lydia was born. Since Collins was about ten when Lydia was born, you could either move up the date whence they treated Collins as the actual heir, or you could go with canon and have Collins' circumstances change when he's about 15/16. I'd imagine that before that date, they'd intend all his education to be for the purpose of his future independence, so they'd either prepare him for an eventual position in the church, or maybe (if you want to go this way) there is an informal arrangement with Mr Philips that he'll one day take over the law practice (since the Philips' don't seem to have children). Then, when the Bennets realise there will not be a son, they'd of course change course and prepare him to take over the estate one day. I'd imagine there'd also be a great change in Mrs Bennet's attitude towards him. I can see her as previously treating him a bit like a burden on their family, emphasising all that they've done for him (and perhaps young Collins develops a tendre for young Jane, depending on when you decide to make a change, and Mrs Bennet quickly dissuades him of that notion, because Jane can do better), and then when they realise Collins will eventually inherit, I can see Mrs Bennet as sucking up to him and telling him he always was like a son to her etc. Or, Mrs Bennet being as she is, it could also go vice versa, with Mrs Bennet first being very sympathetic to the poor orphan boy and then making a 180 turn and accusing him of wanting to steal the inheritance from poor dear Jane and her poor sisters.
Just some ideas, I don't know which way you intend your story to go ...
Just some ideas, I don't know which way you intend your story to go ...