For our situation, we were very proactive as parents as soon as each of our sons showed delays and signs and symptoms that appeared atypical. We wanted to know if some condition was involved, so as we could determine how to best handle that. We kept pushing the medical community for answers, and not giving up until we got answers, as we felt putting our head in the sand (being in denial) was uncaring and irresponsible to do.
Although each of our children had sime similar issues regarding speech delays, feeding and sleep difficulties, and sensory sensitivities. this differed in terms of severity and how they showed itself for each of our sons. As well, each of our their personalities seemed to differ, and they each had other issues that were unique. One was more hyperactive and with more sensitivities, and the other had more need for routines and more early behavioral issues.
The point I wanted to make is: We feel each child should be given the same efforts, respect and love, but as we feel each child with Autism is also different, there will be times it will be ok to treat them differently, as they are different, and as each has different abilities, limitations, preferences and needs. So, in the situation of the original poster, the parents we feel should have made the same efforts and showed the same love, but if anything, given more focus and not less to the one with more needs, but equal attention and love. The parents messed up there.
Any number of reasons could explain the parental behavior. They maybe assumed because you were a girl you could blend in and learn to socialize and conform to behaving like your peers. Or maybe subconsciously they did not worry as much if you succeeded as you were a female, and with them thinking you need not be successful at work in the future, so no need for a diagnosis and to consider treatment. Or maybe they did not want that stigma of Autism, once they learned about the Aspergers for your brother. Or sometimes parents show favoritism, towards a son or daughter.
Whatever the reason(s), we as parents never understood parents like that. We see both of our sons as equal in terms of greatness, regardless of their differing personalities and severity level of their conditions, as each has strengths and needs, and we each try to tailor our assistance accordingly and show our appreciation accordingly. One with more abilities does not mean better, and one with more needs does not mean worse. Unfortunately, most parents either seem to want to compete with others to who has the best children, or with pressure to change their child to fit in, or to excel beyond their capabilities , or the opposite: they may want to hide certain things and hope they are not noticed or go away.