She is a tailor and tells her customers to buy from certain shops in return for a percentage

There is nothing wrong with telling people to go to certain fabric shops in return for a certain fee or a certain percentage of what the customers buy from these shops. The fuqaha’ regarded it as permissible to accept payment for acting as a middleman or broker, and there is nothing wrong with it being a percentage of the price of the goods sold. 

It says in Fataawa al-Lajnah al-Daa’imah (13/131): It is permissible for the middleman to accept a payment of a certain percentage of the price for which the product is sold in return for telling customers about it. The middleman may take his payment from the seller or the buyer according to the agreement, so long as there is no unfairness or harm. End quote. 

See also the answer to question no. 45726. 

It is not permissible for the payment to be unknown, so as to avoid dispute between your sister and the shop owners, if they give her less than she thinks she deserves, unless your sister wants to tell people about stores without any payment, and the shop owner wants to reward her for what she did voluntarily; then he may give her whatever he wants. 

In order for this transaction to be permissible in the first place, there must be sincerity and honesty towards the customers. It is not permissible to send them to these stores if she knows that the same fabric is available in other stores for a cheaper price, or there is better fabric for the same price. 

And Allaah knows best.