I've gone through something like this with my sister … except it wasn't "money doesn't really matter" or a 3rd vehicle …
we she rejected, out of hand, my suggestion … I told her that I'd given her my opinion … and after a bit of silence, I then asked her what it was she wanted me to suggest
she himed and hawed for a bit and finally told me what she wanted … I said fine, I've told you my opinion … you're going to do what you want anyway, why'd you call ? eventually she calmed down, and asked if I really meant what I'd suggested … she still got what she had wanted all along … shrug