Covert incest. When a parent uses a child to fulfil an adult emotional support role that should be filled by another adult; partner, friend, family or therapist.
But in this case, the incest is overt, not covert. The woman is saying out loud on social media that her 8 year-old daughter is her deceased husband and that the mother relies on the little girl for the sort of love, care, understanding, connection, support and practical assistance that adults customarily get - or hope to get from other adults who are partners, spouses or romantic love interests - or who are in the other types of roles you mention, or are hired helpers like home health aides, personal assistants and domestic servants.
Emotional incest isn't necessarily covert incest. In fact, the opposite is frequently the case. Emotional incest is often very blatant and public. Many abusers do it out in the open not just where others can - put so others can see. People who engage in emotional incest frequently flaunt these types of inappropriate relationships because they want others to witness and know what they are doing.
If not literal, certainly emotionally incestuous. This is not a mother acting like a parent to a child.
That was my first thought (CONCERN).
This is child abuse full stop. The incestuous nature of this post makes me sick.