I'm having a hard time listening to God. Some say you hear Him from you heart. But my sister hears voices from the brain. So, How can I help her too?
There could be an underlying problem blocking you from hearing Him clearly. Satan could be trying to block you as well as he is the prince and power of the air. (Ephesians 2:2) You can pray and ask God to cleanse anything out of you from something eaten, repair anything in you that was damaged or remove anything in your life that is blocking your ability to hear Him completely. The pollution in certain foods and the water supply causes some of our internal organs from properly functioning clogging them up and prevents us from hearing Him clearly. Aluminum, fluoride, sodium nitrates, corrosive preservatives such as TSP in cereal are just a few. Certain commercials, secular music, television shows and movies have subliminal frequencies and messages that will clog up your receptor also. (Psalm 101:3, Matthew 6:22-23) It's why God told us to eat specific things, be careful of what we see and wear certain materials of clothing. It could also be an underlying / unknown self-sin or ancestral/generational sin/curse. (This part requires a more in-depth discussion)
Static electricity will hinder communication from Him such as using plastic and metal combs or brushes (do not use boar bristle ever). Try wooden brushes and combs as they produce the least amount of static. Certain clothing that causes more static electricity will block it also. Linen was suggested to Levite priests because they do not cause static electricity and have different properties no other material has. Cutting our hair too short will also do the same as it's part of our nervous system.
We also have to repent of anything done that day and cover ourselves with the Blood of Jesus before entering into His presence. If your sister can hear him from the brain, it's possible she has a prophetic gift and calling on her life. Sometimes feeding upon the word more / reading the Bible will unclog it also as it builds up your Spirit. Having more devotional time spent with God works also as there is really no limit. The more you draw near, the more He wants to be with you. (James 4:8)
Everyone has the ability to hear Him as long as they come to Him. (John 10:27) Those with special callings can hear from Him at birth. He does speak in different ways to everyone to prevent the enemy from intercepting messages so don't be discouraged and continue trying, praying and persevering.
Also try Psalms 100, 149 and 150, through song, praise and worship as well. Singing these three will help as they are part of open heaven prayers.
God said to give you Malachi 3. He didn't specify which verses so just ask Him to speak to you from what you find in the chapter.