I think it really depends. People all have different types of relationships with their family. For me, I was happy to come out to my mother. Of course, it was freeing as getting away from my mother as a child and as a young adult was difficult. She was abusive, full on Mommie Dearest and then some. So DH and I would actually enjoy telling her that not only are we the only stable relationship she's ever seen, only stable child she has, but we are comfortable with poly and not being straight. (Just to see that twitch when she knows she can't say anything back.)
Meanwhile, there are family I have not come out to at all. I know they are conservative and love me and would try to accept it all just as they have tried to accept other decisions they may not have agreed with. Personally, I don't feel a need to come out to them as they are not doing well and may not be long for this world. Considering how little impact my other relationship has on them, not going to worry about it.
"Coming Out" whether about poly or sexuality or kink even, isn't something you just do in one fell blow. It's a stages thing. Who you come out to, when, how much. There's a lot more to it. If you feel it's important to let family know, start simply. If they already know you are in a relationship and are assuming it's a committed one, they are probably assuming monogamous. Sadly, that's just how it is. The biggest hurdle I see people go through is explaining how you can be in a committed relationship, but NOT monogamous. It's like misfires in their brain!
So maybe just start with how the two of you have realized you love each other, and that you enjoy being able to share that love. Sometimes together in your relationship, sometimes in other relationships. You are still figuring things out, but you are amazed at how comfortable you both are with being able to express your feelings for people openly and honestly.
There will be questions of course, up to you how much detail you give from there.
Me: Late 30s pansexual poly. DH: My husband of 17 yrs and father of children. DC: LDR of +1 year