Assomigliare is similar to piacere in that it's intransitive in Italian and transitive in English: it's "to be similar", so more or less "the niece is similar to her aunt" - the Italian 'a' has the same function as the English 'to'.
I was thinking that the use of "a" here is simply as a link to relate the two parts of the sentence ie prepositional. Isn't "sua zia" a direct object in the above sentence? In which case the verb assomigliare is being used transitively in this case? (To aid understanding of this discussion for others - some Italian verbs are intransitive, not taking a direct object, and govern the use of "a" and "di" after them eg "assomigliare a qualcuno" = To look like someone.) I'd appreciate your further clarification :)
To be honest I think I made a mistake considering "to look like" transitive, the two verbs are probably similar in that regard; in Italian when a preposition is used the object is always indirect (the reverse is usually true except with clitic pronouns), but I'm not sure how it is in English (for instance the Merriam-Webster considers "we look to have a good year" transitive and "he looks the other way" intransitive).
The transitive usage of "somigliare" according to the dictionary was to compare, to liken, so "assomigliare qualcuno/qualcosa a qualcuno/qualcosa" meant "to liken someone/something to someone/something". That usage is archaic and literary. Nowadays the verb "assomigliare" is considered intransitive, but it's true that if you consider the verb+preposition (something Italian grammars don't usually do) the usage of "assomigliare a" is definitely transitive.