I think the OP's partner is a good man, and if I were his friend, I'd probably be telling him to dump the OP.
Losing a parent is one of the most painful things you can experience, and the poor girl is only 9. OP, I suspect you think she should be over it by now, and it's high time for you to meet her. I doubt the child is over it. I doubt your partner is over it. This is a special tradition they have built in dealing with their lifelong grief, and you have no place in it.
It's one thing to want to meet the family. It's another to insert yourself into sacred traditions like these.
And TBH, if you keep being this pushy, you'll never meet her, as you're currently a giant walking red flag, and he's going to be very careful about introducing adults to her who are red flags.
I think you should take a step back, reflect on your actions and apologise to your partner. Explain that you realise how it might have come across - you just wanted to meet the important people in his life - but you understand and respect why that trip is so special. If there's ever a chance to meet her outside of that trip, you'd love to, but you understand that a little girl without her mum absolutely has to have her feelings prioritised.
And then ask if he'd like to talk about his sister - and say if he doesn't want to talk about her now, you're always there for when he does want to share any memories of someone who clearly meant the world to him. You wish you could have met her, but as that can't happen, you'd love to learn about her through his stories.