Sister Advice

My sister is going through something and well I do not know the correct advice to give her. My older sisters husband has been sending off vibes to my younger sister. He messaged her this past Saturday stuff like beautiful, shorty 🔥 and just flirting messages. My older sister lives with him and his family they are married and have a 1 year old baby together. My older sister isn’t working she’s a stay at home mom because of my nephew. My younger sister the one that has been receiving messages from my older sisters husband doesn’t know what to do. She is afraid of the drama and the relationship that will break. She does not want to say anything about it because she is scared of what it’s gonna do to my older sister and her family. She’s thinking about our nephew. So she wants to stay quite and if it happens again she wants to say something.

I had told her personally if my significant other were to do that I would like to know.

What do you guys think is the best thing my younger sister should do?