This might be a bit long, but I don't want to dripfeed. I have changed some details to make it less identifiable, but nothing important.
I am part of a large circle of friends. Some are close friends and then others are more acquaintances. We sometimes socialise as smaller groups, but all get together for parties and things.
I'm a fair bit younger than most of the others, and at a different stage in my life. I became good friends with a neighbour when I moved to a new town, and she introduced me to the group. We get on well despite the age differences, but some of the older friends talk down to me sometimes.
There is one woman in particular who I struggle with. I only see her at larger group gatherings. She always seems to make a beeline for me, only to then ignore me 
she also often interrupts me when I'm having a conversation. E.g. someone asked me a question about a topic I know a lot about, this woman's child knows a bit about so she interrupted me and started talking about her child's experience.
Another time we were stood in a group and she just came and stood right in front of me, so that I could no longer participate in the conversation I thought she maybe hadn't seen me so tried to nudge back in, but she moved again to block me. It was at a wake so I didn't want to make a scene, and I don't really know I should say anyway.
This sort of thing has happened many times. I cannot think of anything I have done to offend her. She has treated me this way since we met. I do not understand why she deliberately comes to sit next to me, sometimes she has actually asked someone to move, only to then ignore me and interrupt anything I say, or make snide comments about things I say.
I've tried carrying on talking when she starts talking over me, but she just carries on as well and no one can hear anything. Also I've tried waiting til she's finished and then saying "As I was saying..." but then she just interrupts again.
So as not to dripfeed, this woman has a terminal illness. She is not actually unwell with it at the moment, but has a shortened life expectancy. However, I do not believe that has anything to do with her treatment of me. Her personality is not affected by the illness and has not changed since diagnosis. And the behaviour seems to be very much directed at me personally.
Obviously I don't want to upset her. But this behaviour is really starting to bother me, and puts me off wanting to socialise with my friends when she is there.
So would I be unreasonable to stand up for myself more? And how?!