politeness level of dochirasama desu ka


If speaking to a family member (who you haven't seen for a long time, say over
the phone and you don't know who you're talking to), will dochirasama desu ka
sound too polite, or is it okay to just say dare desu ka?




I don't think a couple of decades are sufficient for family members to switch
to more polite forms of speech. If you used to talk them in casual manner (as
I do with my parents and brothers), you'll be using the similar tone. If you
preferred more formal speech (as I do when talking to my grandparents &
uncles), you'd continue being polite.

Anyway, the situation in the question is puzzling. You know it's a family
member but you don't know who exactly on the phone?

  * If you have no clue who is on the phone, "どちら様ですか" would just be fine. "どなたですか" is less formal but is appropriate. Example: you're getting a call from unfamiliar number, and the caller doesn't tell you who they are.
  * 誰ですか would probably sound too direct and inappropriate. If your relationship with the caller allows the use of this phrase, you might just as well ask "誰？".
  * If I were in the situation (I know it's someone I know but I don't know who), I would avoid direct questions. Because --- I don't know if it's language/culture-specific or universal --- asking "Who?" sounds like I'm being slightly alerted and cautious, and possibly offend the caller ("This guy seems to think I'm suspicious"). 
    * I'd rather mumble （「もしもし，えーっと…？」）and hope that they'll soon realise that I am yet to identify the caller, or more explicitly tell them so: "すみません，声だけでどなたか…(わからないのですが…)" (Sorry, with your voice only, I cannot [identify] who ..)

Not related to the situation in the question, but I'd more frequently use
"お名前を伺ってもよいですか?" (Can I have your name?) rather than "どちら様ですか?" (Who am I
talking to? / Who are you?) when I know what kind of phone call I'm receiving.
Although not always so, "どちら様ですか" sounds like you aren't expecting the call at
all and want to make sure they aren't calling the wrong number.

