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.
Dochirasama desu ka (どちら様ですか)
Is too formal for a family setting. It would be used in extreme formal setting, such as buisness or talking to strangers.
Dare desu ka (誰ですか)
Works completely fine, but it you would like to be just a tad more formal Donata desu ka would be best.
I hope this helps! Please notify me if anything is unclear.