Said child could be dead or missing or the child they replaced could still be there but the family or parents give all their love too the new child which could be adopted or saved or fostered etc any genre is fine

Examples:
bewear of the brothers
Lan chi

Both about parents who replaced their dead girls with these adopted ones and their brothers both hate them well at first at least but I don't really mind what their family thinks of this replacement they can be for or against or everything between

By the way if you don't have one that fits the bill just give me any regular replacement story you know the kind where someone was in love or is in love or just missing a person and is using the main character as a replacement or substitute for said person any genre us fine

Laika in Space

This does not fit the bill, but I do have one replacement story: Brother for Rent

Also Get Back I haven't picked it up since the first few chapters (bc I'm lazy lol) but I'm pretty sure it had some parts that you might be looking for

but they do not give any sort of love to their new child

Same as above; doesn't fit the bill but has what you're looking for premise-wise: One Armed Amy by Akagi Shun. Its a one shot and pretty short.