You state in dh's culture, he is to provide the christening outfit, candle and a gold cross. Have you discussed this with the parents?
When my daughter was christened or babytized, I provided the outfit. I would not want someone else providing this for me. In our church, (Lutheran), the candle was provided by the church. As for a gold cross on a chain for an infant, I'm sorry, but I see this as a waste of money. There is no way my infant would wear that, (think infant, think getting chain wrapped around baby's neck, not a good combo) and the amount of money you are talking about, I don't see a necklace being a suitable gift for an infant. (wanted to come back after reading all posts...I now can understand gold cross for this christening as you did state they wear them when adults).
But, that part is your choice. The gown and candle you might want to inquire about.
As to the stepsister, no. Unless, she is also getting baptized, I would not provide the same gifts for her. This is the infants day and the stepsister should be old enough to understand that. I might get her a nice book to read that she might like, because I am sure she might like something to do when everyone is congratulating the baby usually at an event after the christening.