Not related, but both great faucets.

I was told at the hardware supply that the owners are (or were) related, and there was a feud between them, hence the 2 companies.

I was told something similar to what scoutcat is saying. Think I was told they were brothers who had a difference of direction for the company to grow. I have a hansgrohe fixture in kitchen and am very pleased with it.

Does Hansgrohe have the same lifetime warranty that Grohe has? And would the Hansgrohe products sold at Costco be the same quality as that you get from a plumbing supply?

The companies are independent so don't necessarily have the same warranties on everything. Whether or not the owners are related.../shrug.

Check the product numbers to see if they are the same from different retailers. The plumbing supply places will tell you theirs is the better product, but often the products are the same. Costco does sometimes get special stuff (their KA stand mixer in a weird size for example), but they don't tend to get inferior stuff.

The split happened like a century ago - so don't read anything into it. The have nothing to do with each other.

Hansgrohe is now owned by Delta / MASCO the giant American building products co. and you will see many of HG's features trickle down to Delta's line. Ever wonder how Delta got those rubber rub-it nozzles on their shower heads ??? Robbed , I mean borrowed, I mean shared the hansgrohe partsbin. Shower valves/trims are supposed to be cross compatible as well.

Products with the same part numbers are the same regardless of the outlet. Now, HG does a better job at mixing up the #'s so a Costco faucet is only at COSTCO as far as I know. That said I've seen and used both and I cannot tell a difference, if their even is one.