Am I overreacting?
Do you all feel like this is pretty normal 8 year old behavior? When my step daughter comes home from school, litteraly the first thing she does is look around to see if my 2 year old has anything new (we run errands often, she'll usually have a cheap $1 gummy ball or something I give her to entertain her on the car ride) and will continually bug me about letting her play with it even when her sister doesn't want her to. If I get them both a cheap toy she'll put hers up in her room and come out to take her sisters. If she gets something new she'll refuse to let the other kids play with it and either close herself in her bedroom, or hid it (and never even open or play with it) . If either of her younger siblings get something new and don't want anyone to play with it until they've had a chance to play awhile, she'll continually bug them and weedle her way into getting them to give it up and I have to continually tell her to knock it off and leave them alone. She's litteraly like the most entitled kid I've met. She has a room full of toys she doesn't play with, she only wants to play with her 2 year old half-siblings toys until they get broke/ lost and it drives me insane. I litteraly have a box of my bios things I have to put up everytime I leave the house or else they get broke/lost/stolen.
It's not normal 8-year-old
It's not normal 8-year-old behavior per se, but in my experience, speaking as the youngest of 4, it's pretty typical oldest sibling behavior. Keep setting limits on it so she doesn't bully the younger ones.
So Normal!!
Anyone who has siblings knows the constant annoyance of sharing. Your siblings often have something new and you don't (that's life!). You have something new and exciting and you HAVE to share. Kids have so little power, that it's no wonder they try to find some areas of their lives they're in charge!
In our family, especially for birthdays or Christmas!!!, you couldn't even ASK to try a toy for 24 hours. The birthday child was allowed to offer if they felt like it, but do not ask to touch, play with or do anything more than admire that new toy for at least 24 hours. It gave the birthday child a bit of breathing room to enjoy their new toy all by themselves. Which made sharing a choice, not an obligation.
I'd say you have two issues here. One - why do you get your litle one a little toy often when you go out? Surely s/he has enough little toys at home to bring one along. Children who are used to getting a toy almost every time they go out (or a treat) become entitled children who EXPECT a toy or treat every time they go out. We learned young, and taught our children the same - when we go out we are running errands. Do not ask for a toy or a treat. It's not happening. Perhaps adopting this yourself will reduce the drama in your house. Of course a child who had to go to school all day is going to be jealous of a child who got to hang out with Mom or Dad, and gets a toy! You couldn't set up a better scenario for jealousy if you tried! So when she comes home your SD is trying to find out what she missed out on. Was it a trip to the store? A trip to a park? New toys?
The second thing is an "obligation" to share. There are some things I never made my kids share. Special stuffed animals, easily breakable items etc. A toy that is difficult to share (a video game with only one player for instance) they got grace on as well. Other things they were expected to share. Because it's an important skill, and because they want to share their siblings toys sometimes too. If they break a toy there should be repercussions (perhaps they have to donate a toy of their siblings choice that is similar in value, or do chores around the house to earn the money to replace it). We didn't allow the choice of "you don't want to share, so you can't play with your siblings toys" because it's possible they would have just chosen to never share in either direction. Which was unacceptable to me. So we made it a rule: in this house we share. And then watched over it to ensure that it was skill they learned, not a punishment of never having anything to call their own.
The non-stop sharing of a sibling sucks! But it's important not to go to either extreme. They should have some areas where they have autonomy, and others where sharing is expected in my opinion.
My SDs, 9 and 6, have little
My SDs, 9 and 6, have little interest in most of the toys I get for DD2. They'll also share their stuff with her unless it's something special to them or inappropriate for her, or she's ruining or playing rough with it. Now, if I got DD something they liked (such as one of those Pop It things) on a day they were with us, they wouldn't be happy that she was getting it and they weren't, so I try not to do that. They wouldn't care if she got a "baby" toy while they were at school and they got nothing, though.
So to me, what your SD is doing doesn't seem totally normal. The one-sidedness seems to go beyond normal sibling stuff. All of the kids in our house get plenty and aren't averse to sharing, so one kid hoarding stuff in her room and never sharing, while expecting others to, seems odd.
I can see this with siblings
I can see this with siblings that have a smaller age difference, but 8 year old and 2 year old? No, not normal. I remember being competitve with my sibling who was 2 years younger, not the one that was 5 years younger, but not to the extreme that you describe. She sounds like a bullying brat. I would nip it before she even starts or send her to her room right away. And a 2 year old and 8 year old shouldn't be getting the same types of toys anyway.
6 years is the same age
6 years is the same age difference between and my younger brother. i am 8 years older than the baby though he passed 48 years ago. My brother and I were BFFs even with the 6 year age difference. Our parents did not allow fighting. PERIOD!
If there are strong parents overseeing the sibling relationships, any age difference can work.
IMHO of course.