There isn't a relationship more complicated than a relationship between a parent and child, regardless of the child's age. As much as children want to be the apple of their parent's eye, sometimes it's a sibling who gets more of a parent's attention. And there can be hard feelings for the child or children who aren't the favorites.
Parents often don't realize what they're doing when they play favorites. They see that one of the children needs more of something, whether it's time, attention, money, advice, etc., so they give it to them.
Most parents want their children to be equally successful, so they might compensate for the child who seems to be lacking. However, the child or children doing well may very well notice the difference, which can hurt their feelings.
They could feel that they're being punished for doing well. If your children aren't getting the love and attention that they're seeking from you, then it's likely that they'll find out from others, and it could be the wrong people who don't have their best intentions at heart.
If you choose to talk to your parent about what you're noticing, it's important that you talk about your observations when you're calm. Hauling off and saying something like, "There you go again, you jump as soon as my sister calls you," isn't going to get you very far. Instead, you can say something like, "I notice when my sister calls you, you drop everything for them. You may not realize it. It hurts my feelings when you do because… and it makes me feel…" Then ask them to help you understand what's happening to them. Tell them what you'd like to see differently.
It can be crushing that nothing changes no matter how many times you've pointed out that your parent shows favoritism to your siblings. For some, it's enough to keep their distance from their parents because you might question whether your parent loves them. For others, you might want to continue a relationship with your parent. If you do, then realize that there will be moments when your parents will mention your sibling and what they've done for them.
When this happens, you might want to ignore it—no need to draw attention to a conversation you want to extinguish. Talk about things that don't involve their favoritism. Realize that you must love your parent despite preferential treatment of your sibling or siblings.
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