Nothing works in life if you try to force it too hard. If you go up to someone on the first day, you meet them and say, «You are going to be a very close friend of mine, we’re going to spend every day together, and you will be a bridesmaid at my wedding,» I guarantee that none of those things will happen. It’s normal to have a good feeling about somebody and anticipate having a close friendship before that actually manifests, but if you force a relationship, it won’t develop healthily and naturally. 

If this truth applies to friendships and romantic relationships, it applies to family. When a parent brings someone into the family who is functionally a stranger to their child, it’s totally unreasonable to expect that child to treat that person like a parent. We wouldn’t expect that from adults, so why would we expect that from children?