Is it acceptable for a husband to make the topic of sponsoring an orphan child contingent on the approval of his siblings, and does the reward depend on their opinion, and what can be done to convince the husband that the decision rests solely with the couple?

It is not permissible to sponsor an orphan without the husband's permission and consent. He must be consulted and convinced of the child's presence in their life, while supplicating to Allah to open his heart. There is no harm in consulting relatives. It is obligatory for the woman to observe hijab in front of a non-mahram child after he reaches puberty. This can be avoided through lawful breastfeeding, making him a mahram.