Sister in law has no kids and keeps giving me unsolicited parenting advice.Am I wrong with snapping?

Depends on how you see it. For me, the above that you mentioned sounds quite acceptable to me since she’s just giving advices and not forcing me to do it her way. To counter the first part you can maybe let her know that all kids are different, some might skip crawling and go straight to standing and walking while some just wants to crawl and not stand. Same goes for teeth. Don’t need to feel guilty towards your kid, they will eventually hit it. Also, it really is different with or without a kid. Before I have my own kid, I would probably just advise you based on the internet but now that I have my own kid, I’ll only give you advises based on what my kid went thru instead of what is online. Walkers are not recommended for babies if you Google online but go for it if you want. It’s your kid, your say. Most of the older kids including ourselves most probably grew up in walkers too I guess? However if you still feel very offended, then just tell her “it’s coming to the age where you should get married or have a kid.” Guaranteed no more future comments.