"My brother decided everything"
Translation:Min bror bestämde allt
Besluta is to make a decision, preferrably considering the options and reaching a conclusion, while bestämma is to just decide and might imply the decision is a bit arbitrary and/or non-democratic. But the nuances are quite subtle.
By the way, the word besluta is a bit peculiar, in that it can be conjugated both strongly (beslöt) and weakly (beslutade) and they both sound right.
We have some words like that in english, for example, spelled and spelt (maybe we have more words like this than you, because it doesn't feel like a particularly unusually occurence to me in english). Then we have some other words where that past and present tenses are exactly the same, like set or put.