The DNA of siblings (brothers AND sisters) can never be more than half identical as around 50% comes from each parent- so they could share between 37.5 and 51% of their DNA. They should share less than 49% if their mothers were sisters but then they'd be cousins (who share an average of 12.5% when their mothers are NOT related), not siblings). Have a look on some genealogy sites to find the approximate percentages of DNA people can have in common for a range of relationships. Plus, remember that percentages are just a rough guide and in real life there can be quite a wide range of DNA shared.
The DNA of siblings (brothers AND sisters) can never be more than half identical as around 50% comes from each parent- so they could share between 37.5 and 51% of their DNA. They should share less than 49% if their mothers were sisters but then they'd be cousins (who share an average of 12.5% when their mothers are NOT related), not siblings). Have a look on some genealogy sites to find the approximate percentages of DNA people can have in common for a range of relationships. Plus, remember that percentages are just a rough guide and in real life there can be quite a wide range of DNA shared.