You will never go wrong using either of these standard forms:
- Si tu hermana me llamara, te lo diría.
- Si tu hermana me llamase, te lo diría.
That pair are completely interchangeable: no meaning changes when you switch between –ra and –se in the protasis (the “if” part). However, there are other, rarer scenarios.
The –se forms are always imperfect subjunctives. The –ra forms usually are, but they also have several other non-subjunctive uses you should be aware of, one of which may be operative here. One is common, one is not uncommon, and one is comparatively rare.
Conditionals using –ra instead of –ía
Although the ‑ra forms are normally subjunctive, they actually can be conditional — and in some cases even the indicative past perfect.
- Quisiera un gran abrazo.
- ¿Me lo pudieras explicar?
- Debieras decírselo.
Notice those are in main clauses, not subordinate ones. They are polite conditionals, not subjunctives at all. And they are perfectly common.
What’s happening is that one can use an ‑ra form, especially with verbs like haber, deber, poder, in place of the conditional. That does not mean that the second half has become subjunctive. It is still conditional.
Con los verbos querer, haber, deber, poder y valer es frecuente el empleo de la forma en –ra sustituyendo en el verbo principal a los condicionales simple (–ría) y compuesto (habría...), así como al presente de indicativo sin cambio de significado.
This happens quiet often with those verbs. Now, the ‑se forms are always, always subjunctives, so they can only occur in the protasis, never in the apodosis. With the verbs listed above, you could actually have them in the apodosis if the protasis were in the past subjunctive. See here for more examples of this. These are examples from that page:
- Si en el seno de algún pueblo católico cundiera tan abominable vicio, se estremecieran de horror aun las potestades del infierno.
- Qué tonto hubiera andado yo, si hubiera escogido en albricias los despojos de la primera aventura.
- Si la carretera estuviese buena, debieras llegar en dos horas.
That doesn’t necessarily mean that those are all advisable or equally acceptable to all speakers. But they certainly do occur.
There is even an example there mixing them around a bit:
- Si pidieras que lo hiciese, yo lo haría.
That one sounds just fine to me, and I rather like the –ra/–se contrast the way it is used there. There you have both forms of imperfect subjunctive in the protasis, and the normal conditional in the apodosis. It wouldn’t sound right to put an –ra form in that one’s apodosis.
Indicative uses of ‑ra forms for pluperfect
Something that also happens but far more rarely is literary uses of the –ra form instead of the past perfect. That does not mean that those become subjunctive. It is a simple analytic tense, not a synthetic one with an auxiliary.
It comes straight out of Latin. You're most apt to come across it either in the written language of literature or else in Castilian speakers from the northwest of the Iberian Peninsula under the influence of Asturian and Galician, two languages in which it is unremarkable to still use ‑ra forms as simple (not compound) past perfects in regular speech.
So far example, the analytic past perfect quedara would there substitute for the synthetic (compound) past perfect había quedado with no change in meaning. It is an indicative use, but it is of a literary register.
I don’t know that I’ve every heard it spoken spontaneously myself. But you do come upon it from time to time if you read literature, especially but not uniquely older literature of a certain register.