I would probably try to feel out her thoughts on the situation, then decide on what to say--and more important, how much to say. I would be as tight lipped as possible if it's clear she just wants to convert you. The more you say, the more difficult it will be. This is the surest way of avoiding a heated confrontation and minimizing hurt feelings.
When she pops the question, you could tactfully ask her what she thinks about your inactivity. You may have to ask several questions to draw her out. This will help you to see where she's coming from, and you could follow up with other comments that could spin the conversation in your favor. If she's clearly determined to convert you, then you could start telling her you'd rather avoid the subject because it will ruin your time together. Cut it as short as you can without getting hostile and be wary of any attempts to draw you into a discussion. Otherwise, someone will say something hurtful and your week together could be a long one. Be as kind as you can in resisting her attempts to convert you, and don't let her make you feel guilty or react if she threatens you with Armageddon. If things get awkward, keep going back to your reasons for avoiding this conversation in the first place. You may have to remind her that she's the one he started this if she gets upset with you, and site it as another reason to avoid this subject. Note that if she gets insistent, she is being very disrespectful of you as a person and you have a right to discourage any conversion impulse she may have. If she tells you she has to do this anyway because Jehovah wants her to, explain to her that this is not an acceptable excuse. She should take responsibility for her own actions and not blame Jehovah. She is the one making this hard, not you.
Bear in mind that she knows you aren't going to meetings, so she obviously knows something is wrong. She will at least have a suspicion that you have doubts about "The Truth." I don't know your sister, but the standard motive for questioning an inactive JW is to bring them back into the fold. The more information you give her, the more you are inviting her to dig in and try to get you to come back. The goal is to avoid ruining your visit, not to convince her you are right or to make her understand (unless she's a very liberal Witness). If she's a diehard Witness, you will only be wasting your time. Sad, but true.
IsaacJ