Both use the same word, heos, for "until."
Samuel was written in Hebrew.
Matthew was written in Greek. (At least our translation; some claim there is also a Hebrew version. As a NATIVE (I am a Jewish covert to Christianity) Hebrew speaker (albeit modern Hebrew), I would disagree.
The long and short is that there is little or no Biblical evidence of the eternal-virgin issue. Indeed, the little that exist to address the issue --- complex excuses nothwithstanding --- are contrary to the position.
Basically, one can parse "brother" fifteen ways and "until" and "firstborn" as the roman church does. Any one of these, standing alone would be defensible. Put together, however, it is almost certainly contrary to the Word.
Moreover, there are numerous early scholars (including the Roman called the "Father of the Western Church" whose name I forget) circa 200AD who rejected such claims.
(BTW: Luther was rumored to reject this claim, but publically backpeddled, as he was facing certain death for heresy. Means little to me, as Luther is just another flawed man.)
In addition to the lack of real Biblical evidence, a the logical reason behind the "eternal virgin" claim exists, and has been documented as such since the claims was made ---- it was to compete with the pagan practices of the Romans (Diana --- the "Mother of God") and make the natives more receptive to the hard medicine of the Gospel.
Co-opting pagan practices was a repeated strategy of many churchs --- from Christmas trees, to Jesus as part of the Hindu Pantheon, to the paganesque worship of Christ in Central America where little more has been done than change the name of the pagan "gods" to saints.
Whatever gets the job done, I guess.
And, again, I DON'T CARE. What matters is Christ.
As such, I wish you the best, and ask only that you remember that Christ is the ONLY way to heaven.