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The original was posted on /r/amitheasshole by /u/Ok_Hovercraft286 on 2024-01-11 11:45:27+00:00.
SIL married my brother 10 years ago. They each had a daughter (both 18 now) from different relationships. My niece was really excited by the idea of a sibling and a bigger family. She used to ask her dad to give her a sibling when she was still in preschool and while she did stop asking, the wish remained. The fact she thought step-niece was the coolest and best person ever made the excitement more palpable. But it was clear from very early on that step-niece did not view my niece in the same kind of light. The girls were very different and very early on my niece became very demanding of step-niece's time and SIL and my brother were very eager to see them spend time together so they encouraged niece in her demandingness and would scold step-niece for wanting to put friends over her sister.
I became concerned about how demanding my niece was being. She wouldn't even ask step-niece if she wanted to do something, she would tell her, and neither my brother or SIL batted an eyelid. Step-niece would say no and my niece would tell her no was not an option. So I spoke about my concerns to my brother and he told me he knew it was going to push his stepdaughter away in the future but he also knew how much my niece wanted a sister and how much his wife wanted her daughter to accept his as a sister. So he wasn't going to get involved.
A confrontation happened between SIL and step-niece a couple of years ago that the rest of us were made aware of. SIL told step-niece that she would need to get used to having her sister in her life because when she grows up she's going to have to make the time for her and this is good practice for that. Step-niece told SIL that she could just not see her at all and SIL that wasn't a possibility and she would always be part of the family.
Step-niece turned 18 in July and she moved out of my brother and SILs house and has not been to visit or kept in touch with them since. My niece has tried to talk to her over DM or call but she has ignored her and maybe even blocked her in places. The only people this seemed to surprise was SIL and my niece. My niece cried for days about it and said she didn't understand while SIL has grumbled for months about it. But during Christmas it was at an all time bad and she showed up to the family celebration we had on the 30th and was blasting her daughter to everyone and saying how surprising her actions were. A lot of the family moved away from her but when she sat down next to me to do her vent, I told her only she and my niece were surprised and the rest of us saw her daughter's actions coming a mile off. I brought up that step-niece had even told her what she'd do 2 years ago and she didn't believe her.
SIL called me insensitive and told me nobody could have known a teenage girl was serious when she was having her little temper tantrum. She said most kids would never want to be away from their siblings or their families like that and it's cruel for me to lie and claim otherwise. AITA?