"¡Hola, Valeria!"
If I may join in, I'd say here we have a "se" which replaces "le", an indirect object (IO) pronoun. In the sentence you gave, we use "se" to avoid having *"le las"---where "l" is repeated, which would not sound good to us Spanish speakers! :)
So, to explain that in more detail, let's start with the full sentence, with no object pronouns:
(1) "Ramiro ha robado las joyas a Sira."
Now, let's replace only the direct object (DO), "las joyas", with its corresponding pronoun, "LAS":
(2) "Ramiro LAS ha robado a Sira."
Next, let's also replace the indirect object (IO), "a Sira", with its corresponding pronoun, "LE":
(3) "Ramiro *LE LAS ha robado." --> "LE" + "LA" are combined here, but "le la" doesn't sound good...
Therefore, we simply replace "LE", in (3), with "SE". We get sentence 4 below (which is the sentence you gave us!):
(4) "Ramiro SE LAS ha robado."
So, in summary, "SE" in (4) is a "variant" or alternate word for "le". As you will gather, "se" is not a reflexive pronoun here. In fact, "se" is used in many different ways in Spanish---which I'd say makes this little word one of the most difficult in Spanish! :)
I hope that helps!
Alfonso