How common is this?

not sure about the family part, but playing with multiple accounts is pretty common.

Accounts for shaving, bubblestrat...

Its against the tos, but I know several folks who share accounts or have multiples.

Personally, I dont, but have considered creating a bubblestrat account.

I don't. My kids play my account with me though. My son has his own but kind of gave up on it.

My BIL will spin stops for his son periodically during the day (there's a stop at his work). On the spectrum of "cheating" I consider this so far down the list as to not register on my give a damn meter.

I ran into a guy today that runs 7 accounts all day...that are his 'kids'.
Now I know who the 'team' is that swoops on gyms from time to time. He had 7 x 3500CP Tyranitars on each of the multiple accounts he placed in gyms.
Then told me he has a team of accounts on my team too and raids gyms with them so his others don't get retribution.
So yes...multi accounting is rampant.
He admitted to at least 10 accounts on multiple teams

Personally, I spin pokestops for my son when I am collecting balls, and will catch the occasional for him - but he likes to get new entries so I can't take that away when I see something.
If I take a gym while stocking up for him, I do not place anything in from his account - I use only my account. On the rare occassions he is with me when I take a gym, he will sometimes place something in...but not always.
I figure if he is actually there and he finds the free spot then he is entitled to take it - but it is rare. He has about 90 pokecoins with zero gym battles.

TL/DR the only thing I will do for a family members account is spin stops when I am at pokestop rich area.
...but multi accounting is rampant.

I usually see a guy with two or three mobiles , it is actually for his sons as I saw them together as well as we only have one rich park in the whole cite , he do every thing for the accounts he use ( battling , pokestops,catching )

I think this is done a lot as I see it on my dog walks, but honestly, this is so far down on the list of bad behavior in this game that I don't care that much since some of it doesn't affect me...

Spinning stops doesn't affect my game, the gym thing can be annoying if there are limited gyms, but if you are high enough level, you avoid both multi-accounters (usually, they are lower level) and gym shavers which are more rampant.

I know of someone who has like 10 accounts on 2 teams and it's sorta annoying, but you don't tend to see them all in gyms.

My thinking is I'm tired to catch stuff on 1 account, why the hell would I want to bother with another?

It just seems like more work...

Oh that's wat happens with my dad and myself. Usually if I haven't fight gyms during that day, he'll help me when I'm sleeping to fight gyms and collect pokestops as well as catch Pokemons for me.

I did that during march to lvl my sons accout past 20 so when we play togheter I use his for prestige and he uses my/main accout.
1 spin, few catches, 1 gym on occation, if available at the time of my action.
When we walk, we use my accout, he when is in the mood or we do an interesting catch, me the grinding :).

If I'm with my dad sometimes he asks me to fight gyms for him on his account, to use up his potions + revives, at least get some XP rather than throwing them away. We're on different teams, but we never shave gyms for each other.

He's actually higher level than me, and has better Pokemon. He caught a wild DB/DC Dragonite that he never uses for attacking and instead uses Vaporeon + Gyarados most of the time, it's actually annoying.

When he times out vs Blissey that usually convinces him to use it.

One of my coworkers does this, tho Ive never seen his son...Son confirmed by others

One of my students' dad does this. His son stopped playing a while ago, but he said he keeps it going so he's not behind when summer break begins. I have no problem with it. At least he's not spoofing.

The other night my husband saw a snorlax on nearby. I grabbed my phone, my husband's phone and my son's phone. I ran, and I'm not a fast runner, to the poke stop, caught the lax for all three of us.
about 99% of the time we play separately, but more than once I've walked for them and spun stops when their item bags were crazy low.

Raised hand.