Peace be unto you also.
I am not arguing that brethren cannot possibly mean 'fellow Israelites', and you are quite right to say that references are found in the Bible which show that. As I said, it is understandable that this is what the Jews believed for generations. However, it cannot be denied that the Ishmaelites are also 'brethren' of the Israelites, and of the seed of Abraham a.s. too.
Well, given the fact that we descend from Adam and Eve, one could argue we are all brothers and sisters.
The first three verses of Deuteronomy 18 outline what the word "brothers" means.
Deuteronomy 18:1-3
18 The Levitical priests, all the tribe of Levi, shall have no portion or inheritance with Israel. They shall eat the Lord's food offerings[a] as their[b] inheritance. 2 They shall have no inheritance among their brothers; the Lord is their inheritance, as he promised them.
Clearly, inheritance among brothers meant inheritance with Israel.
The Jews have also believed that Elijah himself will come down from the heaven/sky, for there is no reference in the prophecy of his return to someone LIKE Elijah, and they insisted on what is in their scripture, not willing to accept the explanation of the fulfillment of the prophecy in a symbolic or metaphorical way, and this became a reason to reject Jesus outright.
Jesus didn't claim He was Elijah. He claimed John the Baptist is.
And the Ishmaelites are not brethren in a symbolic way alone, for it could be argued that those Ishmaelites who believe in the Biblical prophets and their message are also close brethren in faith to the Israelites. They are not idolators like the foreigners around the Israelites were. However, it is not merely such a meaning of 'brethren' that is being referred to. It is not a stretch to say that the Israelites are brethren of Israelites.
Matthew 17:1-13
17 And after six days Jesus took with him Peter and James, and John his brother, and led them up a high mountain by themselves. 2 And he was transfigured before them, and his face shone like the sun, and his clothes became white as light. 3 And behold, there appeared to them Moses and Elijah, talking with him. 4 And Peter said to Jesus, Lord, it is good that we are here. If you wish, I will make three tents here, one for you and one for Moses and one for Elijah. 5 He was still speaking when, behold, a bright cloud overshadowed them, and a voice from the cloud said, This is my beloved Son,[a] with whom I am well pleased; listen to him. 6 When the disciples heard this, they fell on their faces and were terrified. 7 But Jesus came and touched them, saying, Rise, and have no fear. 8 And when they lifted up their eyes, they saw no one but Jesus only.
9 And as they were coming down the mountain, Jesus commanded them, Tell no one the vision, until the Son of Man is raised from the dead. 10 And the disciples asked him, Then why do the scribes say that first Elijah must come? 11 He answered, Elijah does come, and he will restore all things. 12 But I tell you that Elijah has already come, and they did not recognize him, but did to him whatever they pleased. So also the Son of Man will certainly suffer at their hands. 13 Then the disciples understood that he was speaking to them of John the Baptist.
This explanation should not be objectionable for Christians who even go so far as to say that the prophecy in Isaiah 53 about the suffering servant seeing his offspring is a reference to the spiritual offspring of Jesus, not to his physical offspring. Whereas he already had spiritual offspring in the form of his disciples and many other followers even before the attempted crucifixion, and so the prophecy loses significance if understood in that way.
If you can find me some things in Isaiah 53 that cannot apply to Jesus, please point them out.
I have pointed out that in Deuteronomy 18, it is made very clear that "brothers" are a reference to Israel. If Muhammad was an Israelite, he maybe would have fit into that prophecy. However, that isn't the case.
So, why do you give such explanations for other prophecies, but when it comes to Muhammad s.a., you reject the perfectly valid explanation outright? Is this not similar to what the Jews do when it comes to Jesus?
Peace.
Another reason why I could not accept Muhammad would be that the book he claimed came from God to Him directly via Gabriel has some errors in it that demonstrate that He is not its author. Of course, I believe that the rejection of Christ's crucifixion and divinity are the biggest errors in the Quran, but to state that Jesus is God is a matter of faith, and many Muslims I speak to reject Tacitus' account.
So there are some more obvious things in there that can be easily proven are not from Him. I brought this up at one point already, in a different thread. I will paste what I wrote earlier.
As for a historical mistake in the Quran, I would advise you to refer to the thread below, which is about the Quran's claim that the Thamud carved their homes out of rock in the mountains, and that their dwellings can be seen and make clear the Quran's account.
http://www.christianforums.com/t7813817/
You can actually see massive buildings carved out of rock at Al Hijr and Petra. However, they were not made by the Thamud before Moses, but rather by the Nabataeans a few centuries before Christ and some time after Him.
If you want a scientific error, refer to the verses 55:19,20.
There are no two seas or bodies of water that meet and do not "transgress".
55:19,20
He released the two seas, meeting [side by side];
Between them is a barrier [so] neither of them transgresses.