1. Remittance needs to comply with relevant laws and regulations: Remittance needs to comply with relevant laws and regulations, such as foreign exchange management regulations. If the remittance amount is large, you need to go through the formalities of foreign exchange approval.

2. Need to comply with anti-money laundering regulations: In order to prevent money laundering, terrorist financing and other illegal acts, banks will conduct anti-money laundering review on remittances. So my sister needs to provide real information about the purpose of remittance and the source of funds.

3. Pay attention to tax issues: If the remittance amount is large, it may involve tax issues. Sisters need to understand the relevant tax regulations and avoid tax problems. In short, it is not illegal for my sister to send money to a foreign brother, but it is necessary to abide by relevant laws, regulations and rules to avoid illegal problems. It is recommended to consult the bank or relevant institutions before remittance to understand the specific remittance process and matters needing attention.