You haven't even been in your current car for a year. It doesn't seem like a good idea to switch unless you can get a really good deal on the M3. I presume that your sister would take over your 328 lease. Would you lease the M3?
I don't want to turn this into one of those "you can't afford it" threads, but you do have to carefully balance your need vs want ratio against your own financial situation.
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