My ten year old has become such a pain! She acts like she forgets everything we ask her to do and is as lazy as possible doing it. She has also become a BIG whiner. She won't try to ride her bike because its "too scary". At the park today she wouldn't try the monkey bars because they were "too hard". I know part of it is sibling rivalry, like today when she asked why I wasn't watching her on the playground and telling her good job like I was her 3 year old brother - I tried to pay more attention but COME ON you are 10! I am not impressed by your walking up some stairs. She continues to try to get praise for 3 year old things when she is 10. We've discussed age appropriateness and all that and when I try to have real discussions with her she gets a blank stare or plays with her hands or some other way of saying "I don't care". I want to spend more individual time with her but she gets me so frustrated and tries to act so dumb I get so annoyed I don't WANT to spend time with her. Everytime I think of this I feel like the worst mom ever, but 3 years in I'm so tired of her saying how life was better before her brother, it was better for her because SHE was a really cool kid, and now she's more often an annoyance.

My kiddo is nine, so a year younger, and she doesn't have a sibling (yet). So, feel free to take what I am saying with a grain of salt.
Can you set aside one afternoon/evening a week where it really is just you two (even when she's being a giant PITA). I know with Zoe, when I am finding her to be particularly annoying, if I spend more time with her she chills out on it, and I do call her out on the annoying stuff.
Maybe you can start focusing on praising her for the stuff she's doing that IS age appropriate. It sounds like she is looking for your approval/attention, and by playing dumb, or bugging you repeatedly, that's how she gets it. So, maybe by giving her positive attention about things that are really positive that she does, she'll stop being annoying.
Hey Katg, we have worked on it and its getting better, but thanks for the ideas. Definitally alone time and praise are helping.