18 USC 921(a)(33)(A) does NOT consider a misdemeanor crime of violence against a parent or sibling as a MCDV (for purposes of lawful firearm/ammunition possession), unless the person who committed the offense was "similarly situated" to the potential offenders who are listed in the statute. For example, a misdemeanor crime of violence by a parent against his or her child is a MCDV under 921. It would therefore seem, for example, that if an older sibling reared a younger sibling as a parent, in the absence of their parent(s), a misdemeanor crime of violence by the older sibling against the younger sibling would be a MCDV under 921, as the older sibling was similarly situated as a parent or guardian of the victim. Please note, however, that this is a federal statute only; each state has its own definitions of crimes involving domestic violence.
Might I just add to the prior poster's answer that your question leaves out the facts of the situation - crimes, while defined by statute are inherently factual in nature; that is, whether or not a crime has been committed sometimes depends upon a single fact. And, interestingly, sometimes what we call "facts" can only be defined with a close familiarity with the case law of a jurisdiction (i.e. the term "foreknowledge" is not always what a layperson might think it is.)
Please, take the time to consult with a lawyer in your jurisdiction so that you can get a legal analysis that takes into account all of the facts of your particular situation.