Must be a small family or a HUGE bathtub.
I don't bathe regularly with my son (or my mum, for that matter), but I DO bathe occasionally with either of them. Also aunts, uncles, cousins, siblings, nieces, nephews, etc. But we don't fit in bathtubs! (Scandanavian, we hardly fit in a tub if we're solo!). Piling in and out of showers, though? Common. Hanging out in the bathroom chatting while someone else is bathing? Even more common.
In my family, there's no nudity taboo. Part of it has to do with about half of us are european, and nearly all of us are athletes. We pile into showers together. Whomever is there. Esp. after sporting events, or at the beach, or camping/hiking. Or if there aren't multiple showerheads (like at home), then we're lining up and dominoing in and out, or one in the shower another in a towel brushing teeth, another changing or hanging out talking.
In Japan (lived there about 1/3 of my life), we used the public baths fairly regularly (imagine a whole series of swimming pools with different temps, some with different scents, some are set up like water parks, some are natural hotsprings both indoors -building built around the natural formation- and outdoors... LOVED it when it snowed! We'd be in the steamy black water, snow ringing a few feet around the edges. As kids we'd hop out and make bum prints!)). Some are split by sexes, many are co-ed. All are nude (you clean yourself ahead of time, soap, all that jazz in the showering room). That's about the only way any of us would fit IN the bath.
So in my family... we see same and opposite sex grandparents until their death.
Birth to death we see people naked.
There's never anything sexual about it (Eeeew, icky, gross! That's family!)