Firstly, my children are 24, 20 and 6. That is a HUGE age difference and so my youngest is an only child because of it. I do not like that they do not want to have anything to do with him, but that is my cross to bear and not my youngest so I deal with it the best I can.
Secondly, there would never be a time when I pay my child to play with a sibling. Never. And a sibling will find time to play with a younger is there isn't any entertainment for the older one. (ie: take away that xbox, tablet and ipod with all the games and suddenly, VOILA younger sibling isn't so bad.
Also, you mention that relatives don't want to take time to take the younger sibling out because he needs more care and attention. I find that offensive and it'll make your younger child act out because he sees his older brother going and getting to do things all the time that he never gets to do.
Since you work from home and your children are there with you, I'd set ground rules for how they are to play and entertain each other while you're working and leave it at that. You will play with your brother. You will not complain and you get an hour on that play station, etc etc etc.
And people, this is not an easy life out there. Making your 9 yr old entertain you 4 yr old so she can work is not the end of the planet. So what if he's the baby sitter. Its called family. They help each other. We are producing a world full of spoiled rotten entitled brats because we all think the world is fair. The 9 yr old has food. The 9 yr old has a roof. the 9 yr old gets to go places and is clothed. He can go and play for a little while with his brother.
I'm sending good thoughts your way.