would catholics welcome islam as brothers in christ if they reject jesus as the son of god but accept mary?

No.

Catholics are Christians. The central tenet of Christian belief is that Jesus was the son of their god.

You seem very confused about the role of Mary in the Catholic Church. Mary is a created human being and is not a deity and is not anywhere near the same level as God. Jesus is God - God the Son - part of the Holy Trinity. So your question doesn't make much sense.

However, the Catechism does state:

"The Church's relationship with the Muslims. "The plan of salvation also includes those who acknowledge the Creator, in the first place amongst whom are the Muslims; these profess to hold the faith of Abraham, and together with us they adore the one, merciful God, mankind's judge on the last day."

No. Not only do we not reject Jesus as the Son of God, whatever lies you've being told, but it wouldn't work fundamentally. A merger with Islam is like suggesting a merger with Judaism. They may be connected, BUT they have fundamental differences. Jews see Jesus as a false prophet, and Jesus is a central part of the Roman Catholic Christian faith. Christians see Mohammad as a false prophet, and in Islam, the believe of Muhammad as a great Prophet is central, so much so that even depicting Muhammad is considered blasphemy. So as you can see, though we are Abrahamic Faiths, we have fundamental differences. But just because we can't merge doesn't mean that we can't have peaceful coexistence, does it?

Our faith is based on that Jesus is our Lord and Son of God (the Holy Trinity), so we cannot call ourselves Catholics if we reject Christ. Neither Muslims nor Catholics accept Blessed Virgin Mary as a god and never will.

Muslims already reject Jesus as the son of God. And if they reaffirmed that position, it wouldn't make Catholics view them as "brothers in Christ." Accepting Mary makes it sound like she's some divine personage, which she isn't in Catholicism. She's not in Islam either, and if Muslims started viewing her as such, they'd be committing shirk.

Catholics aren't as confused as you seem to be about the role of Mary. Muslims already respect Jesus as a prophet and already respect Mary as the mother of a prophet, a holy woman.

muslims believe that jesus is only a prophet , they have great reverence for mary

It would not fly.

That would be similar to telling muslims that Mohammed " was not a Prophet but a charlatan"

I doubt it seriously.