You poor thing. I'm 36 weeks pregnant from ivf so I do understand what it's like and how people can say some truly woeful things, sometimes out of not knowing what to say (in which case please don't say anything!) or outright rudeness. I also get that some ppl just have no idea and really lack empathy and insight. Your SIL sounds like she falls in the rude camp. There is a thread on here somewhere all about the idiotic things say to ppl having ivf which is worth a look.
I would be stewing over those comments and would just really disengage from that relationship as far as possible. Or you could sit her down and lay the facts out and let her know how unhelpful and hurtful her uninformed comments are. I guess it depends what kind of relationship you have with her/want to have.
Wishing you all the best in your ivf journey coming up kitty. It took us 2 yrs of treatments to get our bfp and we are eagerly waiting to meet her any day my advice is to think carefully about who you want knowing as you can't un-tell ppl.