I think it might have more to do with the way that the two of you go about discussing things, more than the topic. NTs generally care about preciseness and pointing out inconsistencies while discussing something. This is purely about the subject matter being discussed and is not a rejection of the person themselves. However, NFs will feel this as a rejection because their ideas and opinions and feelings make a core part of what they are all about. Sometimes an NTs humour can also feel a little unkind for an NF, even if it is very witty and intelligent humour. NFs also have a strong need to be heard and they do not get to the point as concisely as NTs. If you don't hear her out, she will feel dismissed before you have even understood her. NFs often need to talk to process problems. Ts often become impatient with this and feel like the NF is not getting on with solving the problem. Usually NFs can only get on with problem solving once they have rid themselves of the emotional excess they are feeling. NTs often misunderstand this and offer advice too soon. NTs value independence and space and also prefer other people to be able to stand on their own two feet. An NF looks for support and then ventures out more on their own. They need to feel the connection at home base is secure, or they will continue hovering around you trying to establish it. The NTs resulting attempts to get away and not feel smothered only make the NF hang on more before they feel they can take care of their own stuff. NTs often express that NFs are being "illogical" or "oversensitive" because they do not process primarily through their Thinking function. This is offensive to many NFs. Both emotions and logic are needed to arrive at a good decision and it is insulting to be treated as an overtired child etc rather than as an equal. NFs need a lot of appreciation to be verbalized, which NTs don't tend to do. It will really wilt an NF quickly if in addition to no verbalized appreciation, there is also criticism. It makes it even worse if the NT decides they will judge whether the ensuing hurt that the NF experiences is valid or not or if they laugh at/ignore it. The NT ability to distance themselves from an interaction as if they were observing themselves and the other person interacting sometimes leads to finding something funny in the situation when things are at their diciest. It doesn't usually go over well!
Many of the NTs I have talked to enjoy the warmth of NFs and their differing perspectives, but feel that they always have to walk on eggshells which is draining. If you want to have a good relationship with your sister, you will probably have to do most of the adjusting in communicating with her. This of course is a gross generalization as every person is different, but these issues are ones that seem to come up frequently. Ideally if you can work together you both will have a lot of useful things to offer each other. Sometimes it seems a little one-sided as NTs are more independent and often are not looking for guidance or direction. You will have to honestly find some things that you think your sister has to offer that you don't have or you will unknowingly communicate to her that she doesn't have anything you value and she will react emotionally. Hope that help you a little...