Sister offers to breastfeed my LO

So.. A little back story first. I stopped giving my baby breastmilk at 6 months. He's strictly on formula. I was exclusively pumping the entire 6 months. My son was born at 37 weeks, 6 lb 8 oz, and was in the NICU for a few days. He was given formula during that time and never really latched to my breast. No big deal.. I just pumped and he took my breastmilk from the bottle like a champ when I had enough produce.

I started my period just 2 weeks ppd and noticed my milk supply dropped significantly 2 weeks before I started and was low until I got off my period. During the "good weeks", I made enough milk to have some on standby in the freezer. At 6 months, I decided to stop stressing about it and switched to formula.

My son is now 8.5 months old and has been on formula for almost 3 months.

I'm going home to visit family tomorrow, and my sister (who just had a baby and is EBF him) sent me a text asking if I want her to breastfeed my son while we're in town "to plump him up". Then added "how weird would it be if I latched him on my boob? I just picture [my son] getting jealous if he saw [your son] on my boob".

Is this weird that she wants to breastfeed my son?

I'm VERY close with all of my sisters and family, but I just find it very odd that she would come at me with this. Am I overreacting or is this actually odd?

I would understand if I was still trying to give breastmilk to my son and had a low supply.. or if my son actually latched and I was experiencing a dry spell and she was willing to step in and let me catch up on my reserves, but she knows he's been on formula for a while and was never EBF.

I tried to blow it off and not create any animosity...

What do you think? Is it a kind gesture? Or a little weird?