I think i know what your talking about, honestly it does remind me of this manga with the exact same plot just like you described i think its name was I'm a stepmother but i love my family dearly or something like that ╮(￣▽￣)╭
SORRY I got the name wrong its actually I'm a Worthless Stepmother, But I Love My Family Madly so sorry btw but is it this that you're describing here or?
" I'm a Worthless Stepmother, But I Love My Family Madly! " i think you mean this one but you are so right lmao. The plot is basically the same
Istg i read a story exactly like this like i know it has a lot of cliche but this other manga that i read it legit has the exact plot the duke adopting 2 of the children of his dead brother the duke was a war hero the fl has a shitty sister who tries to harm her i can't remember the title of that story it was like duke or something the only difference is in that story the fl marries into the family and in this she gets hired that's all rest everything is same and even the sister in that manga looks exactly like the sister in this one