C.B.
i do think you should let him decide this, and i definitely think it would be ABSOLUTELY wrong to do it behind his back. you use the word baby, so i have to think that until this child is old enough to ask questions, like the girls, you should leave it alone. babies don't need to be introduced to the concept of death and dying and cemetaries. you would be doing this child a disservice. just deal with this situation the best way you can with yourself. when the child is old enough to wonder why mom and dad always talk about this person he has never met, then you can address it. there's a reason we don't introduce this subject to small kids.
i have to wonder about the wisdom of both of you being so wrapped up with someone who has been gone so long, who was gone "long before" you even came into the picture...you mention the girls are old enough to see pictures and ask about them, but never got to meet her. that sounds like it's been awhile. not that there's anything wrong with honoring someone's memory and respecting what you feel would have been their wishes, but there does come a time where we have to move on. this is someone you never even met. i don't think you're being morbid, but i do wonder if your Bf might be unhealthily hanging on to his sister, and expecting your world to involve her a lot more than it should. it sounds like you support him, which is great, but i think it's a little much. just my opinion, maybe there's a lot more to it than i am seeing. but please don't go behind his back and take his baby to a cemetary.