Do not think that this is going to be easy, but you must remember that it does not matter who is right or wrong, getting along is all that matters. This does not mean that you should be a shmatta, but sometimes, just letting go of old issues can help smooth the way for a new and improved relationship. Maybe your daughter-in-law feels uncomfortable because she feels that you are upset with her or maybe she is just a difficult person. Whatever the case, if you want to try to have a better relationship this may be your best bet. If you can try to set aside the past and give your daughter-in-law another chance, you may be pleasantly surprised. It will be crucial to not come across as if you are being the “bigger person,” as this may cause more anger and resentment. Rather, try to be somewhat contrite about the past (e.g., I am sorry if we got off on the wrong foot) and emphasize your wish to begin anew.
If your daughter-in-law continues to treat you poorly after this conversation, you may have to be a bit more distant and just enjoy your relationships with your other children and children-in-law.
Lastly, try to leave your son out of it. He is in an impossible situation and may not know how to navigate the friction between you and his wife. Obviously if he could have fixed the situation, he probably would have already done so. Thus, talking with your son may only further distance you from your daughter-in-law. It is best to speak with your daughter-in-law directly and although it may be hard for you and for her at first, opening up those lines of communication can be the key to a new and improved relationship!
Hatzlacha!