I don't have any experience with the OLSAT, but fwiw my EG ds tested quite a bit lower on the CogAT than he has tested on the WISC, the WJ-III Cognitive Abilities tests, and what seems like 1000 other achievement tests through school. As others have mentioned, this was one test, one day, one moment in time. Also most likely a brief test.
If you are curious and really want to have an idea of what her IQ is, I'd suggest having her tested with either the WISC or another individually administered and widely accepted IQ test. If you're trying to get her into gifted programming at school, and the OLSAT was the screener, you might be able to successfully advocate with a score from the WISC/etc - if it's in the range the school is looking for on the OLSAT.
I 'm not convinced though, that siblings are destined always to be within the same IQ range. Maybe so, but there are so many other ways in which siblings can be different from each other in personality etc, I don't think it's outside the realm of possibility that they may have completely different IQs. Some of what you said seems to possibly match what the OLSAT scores say:
Her younger sister on the other hand, has always seemed more "normal", (7 year old, 2nd grade) with grade level reading, math etc., but very perceptive and mature, with a wicked sense of humor and a good vocabulary.
If the OLSAT is scored with 100 as the median, then "98" is almost right there in the middle of the bell-curve average - and there are probably a lot of "normal" functioning kids out there with smack-dab average ability scores who can also be perceptive and mature with good sense of humor and good vocabulary. I'm not saying your dd isn't gifted - just noticing that with only one test, when you compare it to what you say you've noticed about her in school - the test might actually make sense. If it was me, I'd probably want another more thorough test
Both because I was curious, and if I was hoping she could get into a gifted program within the school district.
Starting to wonder whether she has a learning disability of some sort.
Why do you wonder this? Is it only because of the discrepancy between her results and her sister's scores? Or do you have something else that makes you wonder about the possibility? If it's just due to the discrepancy in test scores, I wouldn't think twice about LDs. If, otoh, you've seen signs you feel your dd is struggling in any area at school (or home), or if you have extended family members who have LDs, then I would dig a little deeper, look at what was tested, do the OLSAT scores show any vairability, talk to her teachers to see if they've seen anything that indicates a challenge... and take her to a neuropsychologist for a full eval that will include individually administered ability and achievement testing.
Best wishes,
polarbear