Correção de frase: ...by the WhatsApp..

Uma vez li que para meios de internet se usava ON mas não sei te informar corretamente ,vou aguardar respostas.

On whatsapp, sem "the".

How about "My sister and I WhatsApp each other every day"?

You may use the term WhatsApp as an informal verb to mean you communicate using the software application called WhatsApp. In pretty much the same manner, you may turn the term Viber (another software application you exchange instant messages or talk through) into yet an other informal verb. It seems to me that, as a verb, email as in "She emailed me out of the blue a few days ago" may have preceded the two aforementioned verbs.

That said, that's sad.

Mispelling alert!

"...yet another informal verb...", not "...yet an other..."

Hey! Here are some suggestions!

1) My sister and I talk to each other over WhatsApp every day.
2) My sister and I talk to each other on WhatsApp every day.
3) My sister and I use WhatsApp to chat every day.
4) My sister and I chat on WhatsApp everyday
5) My sister and I chat over WhatsApp everyday
6) My sister and I WhatsApp every day.

I guess I often use no.2 and 6!
That's it! See you around!

Danilo Adomaitis did provide some nice examples with the word "WhatsApp".

From what I noticed in his example, we can surely dispense with "each other" in the end of it.