E.W.
This is a delicate one. If I were you, I'd speak to both your sister and brother-in-law at the same time...and make the conversation general (rather than directed at the brother-in-law). I think you could easily tie your request to teaching your daughter "stranger danger". Tell your sister and brother-in-law that you are trying something new and wanted to bring them into the loop so that your daughter sees consistency. Then go on to tell them how you are working on introducing the stranger-danger topic.....and have told your daughter that it is not OK for someone to offer her something (like a cookie) and then insist that she do something first (like go with them, give them a hug etc.) You could then say that in presenting the topic, you realized that it's really not OK for even people she knows to offer her a treat in return for her doing something....so to help make it clear for your daughter, you aren't "bribing" her either. Once you've laid the ground-rules, then it'll be easy to enforce them....just a simple reminder to the brother-in-law not to do it. Stress the need for consistency so that your daughter gets a clear message. If your brother-in-law continues to bribe, then a more direct conversation is needed. Whether you speak directly to him or to your sister depends on your relationship and what you're comfortable with.
Good luck.