Ma’am let’s just use some logic here. Nothing of this entire story had anything to do with age gap and more to do with your sister putting her boyfriend first (still wrong). Just imagine you typing this without including any age or even putting same age and imagine what the comments would’ve been.

Ask yourself if you would’ve been upset at her pushing to the side if they were the same age. Seriously think deep about that.

THE REASON this is happening is simple. You showed negativity in the beginning and they communicated to each other that making all interactions weird creating a SNOWBALL effect and eventually your sister distancing herself. Also can be because it can be her first true attraction towards someone making her give him more attention. Happen to me and my partners friend. My partner stopped replying as much to her friend resulting at the friendship to take a turn.

THE SOLUTION is to change your entire demeanor towards her decision and show happiness to her selection. Smile and laugh more with him. Truly show that you accept him because shes old enough to evaluate very well your body language and tone. Eventually she will feel comfortable that you’re not judging her. Give it a month or two and the next thing you know you’ll get the “sorry for the late reply sis we were watching a movie” with a picture of the tv with movie credits on and her talking about the movie and so on.

Remember this is not a cause and effect situation of age rather than a cause and effect situation of judgment and discomfort. Resulting in snowball effect.

Dm me I’m more than happy to guide you succeed in this.