As someone who learned the flute, and who is the father of a professional musician, I would strongly urge the piano as a first instrument. the reason may not be what you would think.
As a flute player, or clarinet, or almost any other instrument, you "see"music as a horizontal stream.
A keyboard player "sees" the entire chord structure--music as a vertical concept, as it were. Notes are related to other notes played at the same time.
Your appreciation of the theory of music will be enhanced as a keyboard player. After you have that understanding, picking up almost any other instrument is easier.

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags