He still complains about it & will often get me up to settle DD or DS back to sleep because they "wont settle for him".

This is what you could maybe explain to Shell???? Not getting in now will only hinder her ability to jump in later, if Jazz isn't getting used to her when she is upset already. In a way she is making you the only parent that can soothe her and have her of a night...
Jazz needs to go to Shell when upset or unsettled, so they can find there own little niche to have.

Shell may regret standing back later on when she only wants you and not her other mummy...