Some people love saying they are alone because they have strong boundaries or because nobody is up to their standards, but usually it is because they are so annoying that nobody wants to hang out with them. This doesn't only apply to couples and single people; it also applies to family members. The fact that you have no family member to hang out with may not be an issue with the family; it may just be a you problem. People don't want to spend time with people who criticize them or mistreat them; it doesn't matter if it is a family member or a friend. If you act this horribly, nobody will ever want to invite you to anything.
In this story, we have a sister who decided to have the worst behavior towards her niece. This is already extremely alarming, as it is a child who is just existing that this aunt is getting bothered by. But then, as if it was not enough, this aunt decided to complain to her sister to also get invited to the trip she's doing with their mom. To be fair, I get her sadness for not having that close of a relationship with her mom and sister; this aunt is alone, but it is all her fault. She decided that her niece is not important to her without any explanation. Which makes me think: why would you want to go on a trip whose sole purpose is to accompany someone you don't like to spend time with? This aunt is creating a mess over nothing just because she is upset that people would prefer a good kid over a childlike adult. Why make a mess over this? Is it just jealousy because they get to spend time together without her? Does she not like the niece just because she's the only one who can act like a child? I don't know, but this aunt's attitude is definitely not okay, and she needs to change it.