The interesting truth about family is that it redefines itself through the years. Loyalties turn to those who are most like family. You mentioned your son. Your sister realizes that your loyalty is to him and in perfect times, his mother, aka your wife.

It'd be great if family never evolved; but, then maybe not. Just as you express needing someone in your corner, your sis has those same needs. The only difference is that she's found them being met by friends with whom she has the most in common.

I know what you're feeling; but, that's one of the stark, cold realities of life. Those you thought would always stand with you, aka siblings, children, etc. will move on to paths suited for their journey.

Peace.