We are enjoined in the Torah to refrain from taking nekama (Lo Tikom)against another Jew. I believe it is Rashi who explains that nekama is defined as (i.e), “You didn’t lend me your rake, so I won’t lend you my rake.” Another halacha related to this is Lo Titor, which is one step further where one tells his fellow Jew, “Even though you didn’t lend me your rake, I will nonetheless lend you mine.” Both are no-nos. BTW, you may not realize WHY your sibs did not make S”B for your child. There could have been extenuating circumstances to which you were not made privy because you were a baal simcha. Something to think about…
So, if you want to make S”B for your niece/nephew, do so with a full heart. Or don’t, but not specifically for the sake of “getting even.” That’s just not a particularly good middah to have or to model for your kids.