Why should Muslims and Christians treat each other as brothers and sisters?

First off Muslims and Christians are not brothers/ sisters. Islam is a knock off of true Christianity. It is actually contradicting to The Bible even though there are some similarities to each religion. God may be the creator of both peoples (Christians and Muslims) but both Christians and Muslims serve a completely different God. Christians serve Christ and Muslims serve Allah. You can't serve to masters you'll either love one or hate the other. Islam premotes Death while Christianity premotes Christ. Christians should obey God and spread the gospel to Muslims so they can see the true light. When a Muslim converts to Christianity then and only then can they live together and treat each other as brother and sister because they live to serve the same God. See KJV Bible for reference.