It is an attempt to control you and get you to do things that your mother wants. Your mother wants to play matchmaker and makeup for all the things she never did right for the family (she is the one who feels guilty). Ask her to show you all of the letters she has recieved from your brother. She doesn't keep in touch and wants to use you to make up for it, but if you do that, it will only be the beginning of a lifetime of nagging and manipulation. She will constantly nag you to bring your brother to her house and play nice tea party "Alice in Wonderland" fantasy games for her family scrapbook so she can show off a "happy family" to her friends. Just be strong and tell her "NO" once and for all. Don't try to justify your reasoning to her and don't let it be a debate. Just be firm and let her know YOU are in control of your life and will not make up for her past.
She has given up trying to get your brother to keep in touch with you and decided that YOU would be easier to manipulate (you're convenient to her, no respect for you).
You are a "treasure" to her (meaning that you come in handy to her when she has use for you, until then, she puts you on a shelf for safekeeping).