Assuming the diagnosis was correct for your SIL’s initial ceebug event the short answer to later events is that the triple shot jabs knocked out at least 15% of the immunity she might have gained from the initial bug. There are lots of examples of this exact circumstance. The next question should be whether the subsequent events are really covid or are reactions to the shots. Remember that testing positive with a nasal swab doesn’t actually confirm covid——it just means one has a dirty nose.
I think it was dumb to even take the shots AFTER you supposedly had Covid. Who really knows if the tests are finding Covid or something else, like the flu or a cold? I don’t trust any of them (doctors) even a little bit anymore.