The Mazda 6 and Mazda MX6 are very different cars. (It wasn't clear from your post whether you knew that.)
Unless your sister is a real car enthusiast, and likes fixing cars, or has a good mechanic, buy her a Mazda 6 and not a Mazda MX6. The MX6 s are great cars for enthusiasts, but are old now, and need some tender loving care.
$3,900 is too much for a 1996 MX6, unless maybe it is in absolutely perfect condition, in which case a real fan of the MX6, and a collector who has some money, might buy it. But you do not tell us what country you are in. You might be talking Australian dollars, for all I know.