It is possible for a German person to invite the mother and father of a Syrian refugee. But this German person cannot apply for the visa. Applicant for the visa is always the person who wants to travel. If an applicant for a tourist visa can show he has been invited by a (German) person who is willing to be a guarantor, then he stands better chances to actually get this visa. Being a guarantor means that you are willing to pay for the costs that might arise from this visit to Germany. This declaration that you are willing to pay for the costs that might arise from this visit to Germany is called "Verpflichtungserklärung". But these days as a Syrian it is very difficult, if not impossible, to get a tourist visa, even with a "Verpflichtungserklärung". The embassies will most likely assume that the applicant does not intend to leave Germany when the visa runs out. They will assume that the applicant wants to stay in Germany and apply for asylum.