Could you get a second opinion from another paediatric dentist? It sounds like she has seen her sister get upset and now has the anxiety of the unknown, even though she's been told otherwise, the feelings are hard to contain. It's hard but id be erring on the side of getting it done sooner rather than later as well, as its related to health. I know for me, I would feel guilty if I didn't do it an it got worse. Having said that, six months isn't long. I would be concerned that it just prolongs the inevitable and results in her learning that it's okay to put things off if you are scared. I would probably lean towards her learning that finding an alternative is important to help you manage the anxiety, rather than avoiding it or putting it off.

I reckon a second opinion, well... Third opinion in this case, might be the way to go.

Being a parent is hard work, isn't it?!?!