"My brother and I have the same mother."
Translation:Min bror och jag har samma mamma.
Why? What's wrong with rhyme? I would not avoid it. 'Mamma' is a familiar way of speaking. 'Mor' is much more formal, but I have startad to use 'mor' much more frequently since my mother turned 80, and I have to talk about her to medical personel, farmacies, etc - it feels much more respectful.
You're looking at it from way too literal an angle. Strictly literal, word-for-word translations don't even work when the languages are close neighbors – try translating a Portuguese novel into Spanish by way of Google Translate and tell me how it works ;)
You can't look at samma and think "this word means 'same'". You have to look at it as "the same". You wouldn't say "the same the mother" any more than you'd say "samma mamman". The definite is already part and parcel of samma. This isn't an exception to the rule, it's how this word works. Rather than an exception, it is the rule.
Detsamma is only used when samma does not stand together with a noun. Also it contains the element det which cannot refer to mamma which is not an ett-word. You could say something like: Han sa [samma sak/detsamma]. which means roughly ’He said [the same thing/the same]’, but you could not say detsamma sak, mixing the two.