Blimey bloss, I have very similar issues with one of my sisters! I won't bore you with the details as this is your thread. Suffice to say she too is superficial, self-centred and unreliable (God, that sounds harsh. She obviously has good points but that's not what we're talking about here).
But I have taken some protective measures. i won't spend more than an afternoon with her at one time, though she often wants us to overnight at her house. Sounds a bit mean, but I know we can keep things lighthearted and manage not to have a bust-up if we keep things short. I don't rely on her, be it for emotional support, turning up when she's agreed to, or doing favours, as she's notorious for promising and not delivering; on one occasion seriously disadvantaging my other sister by doing so. We talk probably every three weeks, and I make sure that I don't let all this silliness get in the way of my relationship with her children. Accepting that she is how she is means I don't feel hurt, or get unduly messed around.