Thinking of GMing my first game...nervous
People on this site join new games every day, and there's a whole host of games to choose from. There's absolutely no reason why they would avoid your game, but even if you get no takers first time around, just revamp your ad and put it out again. Sooner or later you'll get enough players to run. What have you got to lose?
If I can get enough players to run my fringe games, I'm sure you'll have no problems with 'mainstream' Fantasy - you'll probably have to turn some away.
If you're a rookie GM, though, a word of warning from your brief description above - make sure your story isn't just about the second son. I may be teaching my granny to suck eggs here, but...
Players each like to feel that their character is central to the plot. You'll find it much harder to recruit and retain the mature personalities who can handle a supporting role.
You may need a sub-plot for each player with them at its centre. For example, whoever plays the sister might want a Romeo and Juliet love affair with the son of a rival king, a courtier might be scheming to have a knighthood bestowed upon him, a lady in waiting might have her own plans for the second (or first) son's future...
And don't get disheartened by drop outs. It ALWAYS happens. Just recruit replacements quickly and keep running.
Good luck with it. :)