Is anyone else using OPOL - One Parent One Language?

I’m speaking Spanish only to my baby and my sister who left her job to take care of my baby girl while I work and pay her so she doesn’t have to go to daycare speaks only English to her. Her father speaks English as well to her but we recently split up and do not live together.

Anyway, my daughter will be 4 months on Tuesday and I’m thinking about introducing sign language. I have read that it helps making connections. I would do sign language with her and say the word in Spanish and my sister the same but in English.

Any advice or thoughts?