Some of my close relatives keep kosher but not Dvar Charif. In other words, they would cut a raw onion with a meaty knife and later on cut the same onion with a meaty knife to have with a meaty meal. Would their houses be considered non-kosher? Bear in mind I never heard of Dvar Charif until about 12 years ago despite having a religious background and spending time in Yeshiva. As I am Sephardi aren't I inclined to be more lenient as I hear according to Sephardim one should just cut a handbreadth of the onion that was cut with a milky knife in order to use that onion for the opposite type eg. Parve or meaty.
ב"ה Shalom, Although, your fears have a basis, I don't think we have to go so far. First of all, perhaps they do know and you not aware of it. Secondly, not all knives which are used for dairy, are actually milky. To make it milky, it would mean that the knife would have to be used in a fashion in which the knife came in contact with hot milk in a "kli Rishon" meaning a pot on the flame. I'm sure if you look into your own dairy drawer, I'm sure you'll find that true for many of your knives that they never actually became milky. So, although for sure we should not use our dairy knives for meat, or onion, however after the fact in many cases we will discover the knife was not actually milky. Furthermore, according to the Rema (יורה דעה צה:ב) if the majority of a cooked food is a "davar Charif" then we consider it to "revive" the meat or the milk in a pot, but if someone threw in some onions to some sauce and added it to some other food, and the other food is the majority in the pot, then the meat or milk which is absorbed in the pot itself, is not considered revitalized, and consequently it wouldn't be considered a mixture of meat and milk. In addition the meaty food to which some "milky " may have been added, maybe 60 times more than the onion, so the milky onion was annulled. Also, according to Rav Ovadia Yoseph zt"l, (יביע אומר או"ח ח"ח סי' מג) Sefardim came be lenient if a sharp food came in contact with a utensil which hadn't been used for 24 hours. All the best. May you have a happy and kosher Pesach