Why don’t you just show the new partner proof of payment or something from the ivf clinic and politely ask him to explain to his sister he’s seen them and there are no concerns from his end.
I wouldn’t say anything to her at all tbh. It’s not worth the stress on you and you baby arguing with irrelevant people to yours and the babies life. At the stage your at in your relationship if the comments made have bothered you I would just let your partner know this and tell him that if he doesn’t believe things you’ve said to talk to you directly about it and not his family. Good luck with everything I hope motherhood is all you hope it will be and more, you will smash it regardless of who is or isn’t there ❤️x
According to you, the longest you would have known these people is 16 weeks. If you were dating my brother, I would be sceptical of you because I don't know you.