Wow... after reading the OP's latest posts...
Sorry, but, again, I just can't even begin to wrap my head around any of this....
I just can't.
You don't seem to be getting peoples advice or why they are posting their thoughts.
So....
My advice: Tell your SIL, right away, ASAP, that this BF, and/or any other un-invited guests, are not welcomed.
Get out your 'china' and your huge variety of TAPAS, (since, clearly, that is easier than cake on nice/cute disposable paper/plastic party-ware, and obviously one chair is just too much trouble)
NOTE: half of that list would be ten times more than your very limited number of guest might consume. An extra teenaged boy might remedy that.
Enjoy your gathering.
PS: Just a heads-up.... The day might come when your baby gets a little older, and wants to actually have friends and a social life, and they want to go to birthday parties where there will be 'gasp!!!' numbers of shrieking heathen kids running around. You might want to brace yourself. Just sayin'....