Facebook does a lot of network peering though, and has PoPs in Brazil, so there's some potential that Signal goes over transit and WhatsApp goes over peering, but peering is not (or was not, when I was there) a requirement for zero rating. I think AWS may peer in Brazil as well?
WhatsApp likely works better than Signal on congested networks because of the protocol design, timeout length, and DNS fallbacks. These are things Signal could improve on. If your network is so congested that packets are delayed 30-60 seconds and DNS always times out, I expect WhatsApp to still work, but not a whole lot else. That's solvable for other services, but it takes time and determination and accepting the use case as valid and important. Of course, all that and WhatsApp was still inaccessible for a significant amount of time today.
Friends don't ask friends to submit to Facebook surveillance.
Given your refusal, would you think that, for that person in your family, you would fit the description of "someone who can't spend 2 minutes to install a free [..] messaging app to maintain a social or personal connection"?
Would you think that that person in your family would be presumably better off without you?
I use Signal as much as possible, but here in Brazil it seems WhatsApp/Facebook has some deal with all the cellular operators by which the a) give customers "unlimited WhatsApp usage" that doesn't count against their 3 or 5 GB/month internet usage quotas, and b) give WhatsApp traffic preference when their networks are congested, meaning that sometimes Signal messages don't get delivered immediately but WhatsApp messages do. Really.
The "unlimited WhatsApp usage" as part of a basic plan alone means that Signal doesn't have a chance. Most people hit their quota limits during a typical month, and that means no more Signal messages/calls.
I'm sure it's similar in many other countries.