Cimmik is right about に but maybe a little off on おとうとさん.
おとうとさん would not be used to talk about one’s own brother; simply おとうと would be used for that.
Similarly, おとうさん would not be used to refer to one’s own father when speaking to somebody outside the family.
So, the sentence could mean: “(Somebody) lent 10,000 yen to (somebody’s) younger brother.”
Where the two somebody’s could be the same person (as long as it’s not the speaker) or different people (as long as the second somebody isn’t the speaker).