I hate being talked bad about for a hobby that harms no one. So what if they are more for babies/toddlers/children.
It took me a while to really indulge my love of plush animals, because I worried that it was thought of as babyish. Then I gradually stopped caring (as I acquired more plush animals). In the end, I think it boils down to: their opinion of your hobbies doesn't matter. Nobody asked them to share it, and you shouldn't let it affect you negatively.
Be yourself and ignore every negative comment.
If it bothers you, tell someone about it, maybe your parents. I'm sure they would understand and correct your siblings. But if that doesn't turn out well, tell someone you trust about it, someone who will accept you for who you are.
In the end you cannot control everyone around you, but you can control you. Don't let the unfounded negative opinions of others which you cannot control dictate your life which you can.