Is age an issue? Need advice like now!!!!!!?

So my little sister she's 14 and one of her girlfriend who's 19 and married introduced her to a guy who's 17. She confided in me and said that they have been talking and hung out once or twice. and she's starting to develop feelings. Well our mom read her texts and is freaking out because of the age difference. I know he's older but he seems to be an alright guy. And my sister is extremely mature for her age. She's not like most 14 year olds. Does this seem okay? Should i tell her its wrong or give her the okay? shes looking up to me for answers and i dont see a problem. What should i tell her?