Much like Lord Timothy Dexter (see slide 14), Alison here just wanted to know if people cared. So, naturally, the way she went about that was to claim to her closest friends and family that she had cancer and was going to die. For months she updated her nearest and dearest on the status of her condition until, finally, she called everyone posing as her "sister" to inform them that she'd passed on.
You'd think that maybe Alison would wait until the Facebook messages poured in or something to see what people thought about her now that she was gone, but she decided that wasn't enough. Instead, she went to her own funeral and pretended she was Alison's long lost sister, who coincidentally looked exactly like her. People, easily recognizing her, obviously didn't buy it.
Couldn't she have at least worn a wig or something? Seriously, what the hell? If you're going to fake your own death, at least put some effort into it. Weak.