What is my job as a big brother towards my young siblings?

That's a really good question, and good job for asking it!

First of all, you need to be caring and loving (which you must be if you asked this question), and while not suffocating them and being with them all the time, make sure to spend a little time with each of them every so often so that they know and recognize that they mean a lot to you. Also, as obvious as it may seem, don't forget important moments in their lives as acknowledging them and celebrating them is meaningful!

~Renrei Chan

Just play games with them and hang out with them. Make sure you give them their personal space. Dont annoy them.

I guess it might help if you tell your ages. Then I'll edit with my thoughts. :o)