Heart beat

Rebecca • Im 25 years old married and currently pregnant with my second i have had one successful pregnancy and have a beautiful baby boy who just turned 2 in june

Im not trying to be mean at all here and I just want some imput from other mommies but my husbands sister claims.. I mean swears up and down she felt her daughters heartbeat start up at 4 weeks I told her thats impossible seeing as it doesnt start beating till 6 weeks but shes convinced it was the babies heart beat and she got mad at me but still insists she knows what she felt should I prove to her its impossible or just let it be mind you shes 36 and even my husband says shes sounds uneducated everytime she tells someone this again I'm not trying to rain on her parade but who else feels thats impossible?