A and AB blood types from B and O?
It is correct that O- and B+ parents cannot produce biological children with A+ and AB+ blood groups, with the exception of a few rare edge cases. 

Firstly, as noted in the other answer, your grandmother may have the Bombay phenotype. However, this is unlikely, due to the fact that she has been previously hospitalised and it was not noted. 

Another unlikely possibility is that your grandmother is a genetic chimera. This means she would have two sets of genes, and her bone marrow hematopoietic cells had the O- genotype and phenotype, while her reproductive cells had an A allele. 

If we exclude the rare edge cases, then it is strong evidence that your mother and aunt are not the biological children of your grandparents, and you should consider performing genetic testing to verify this if you wish.