Different branches of Christianity believe very different things. For many denominations of Christianity, attendance at a different church would not be a problem theologically speaking. So long as the church believes in basically similar things, then that would be OK.
I grew up ELCA Lutheran, got confirmed Methodist, attended a couple of Baptist and non-denominational churches for a few years, then started attending a Lutheran Brethren church. :-) The biggest doctrinal difference between them was that the Baptists don't do infant baptism. But all shared the beliefs most vital to my faith (role and teachings and resurrection of Jesus). As long as they were professing the same basic tenets of faith, that was fine w/ me. At a glance, these different churches had hugely different types of worship services. Some were huge, some just a dozen people in the congregation. Some were very traditional and some had rock bands playing worship music. Some were much more conservative in dress and conduct. But I could separate myself, my own personal dress "code", my own degree of literalness or lack thereof, my own personal style of music, from that of the church. It was the doctrine that mattered: just what does the church teach about God and the Bible?
Even though all these different sects call themselves "Christians" all variations of Christianity would not be seen as "OK" in the eyes of all denominations. There are some sects which I would not feel comfortable with (I am not comfortable with those which require head-covering for women or which do speaking in tongues and the like) and I know some Christians would be uncomfortable in my church.
In my experience, Evangelicals tend to be more conservative and literal in their interpretation of scripture. Having read the description you posted in the link, my guess is that the doctrine of the Unity church differs too much from her own beliefs (ideas that jumped out at me were the "metaphysical" interpretation of the Bible and the emphasis on living by principles of Christianity rather than emphasis on a personal relationship with Christ and salvation by grace). The latter emphasis (personal relationship, salvation by grace) have been the hallmarks of the Evangelical churchs I have attended. And to an individual who believes that salvation comes through grace and a personal trust in Jesus Christ as one's savior, attending any other worship service might be uncomfortable and feel insincere at best and downright wrong or blasphemous at worst. I am only generalizing and speculating though, so before taking my interpretation as correct, you'd best talk to your sis to find out specifically what she feels about this.
If her faith is vital to her and if her set of beliefs is at odds in some way with the church you're comfortable with, then it seems it would be best to find another alternative. If this is the only church that feels acceptable to you, then perhaps it would be best to politely decline to attend with her. You can make sure she has the opportunity to attend a church and even loan her a vehicle or drop her off and pick her up. Let her know you support her. Just that you are not comfortable with attendance and hope that the two of you can maintain mutual respect for one another's feelings.
I really hope this doesn't become a point of contention between you two. If your sis is like many of the Evangelicals I know, then your salvation is an extremely important matter to her. B/c she loves you, she fears that if you are not saved, you will suffer eternal damnation. And that causes sincere fear and heartbreak to her. It may be something that weighs heavily on her heart all the time. (Again, I can only speculate, but I'll offer the perspective as some food for thought since I know this to be true of some Evangelicals I have met). And I promise I'm not telling you this in an effort to "convert" you, but simply to try to offer you some insight that could make it more comfortable to deal with your sis and to know what to say and how to handle things. Hoping it may make dealing with all of this easier if you just tried to keep that in mind. She's your sister. I assume she loves you. If she's an evangelical, she probably believes it's the single most important thing in your life that you be saved.
If you're not interested in hearing this message, you should let her know now b/c she's likely to continue as long as she has the will. How you talk to her about it is up to you. You can give an ultimatum, but it might really hurt your relationship. Instead, I'd suggest that you try to graciously find a way to express to your sis that you understand and appreciate what she has to say and how she feels, but that you feel stressed and uncomfortable and irritated (or whatever it makes you feel like) when put under religious pressure. If you're OK with it, you can give her your permission to pray for you if she wants to (you may be surprised at how well some people accept this). But let her know if her direct pressure makes you feel driven away more than attracted. And if you feel comfortable doing so, you could share with her whatever spiritual or religious beliefs you have yourself. I'd try to keep from getting into debates though, unless you want to stir the pot.
I dunno if I'm coming across the right way or not. I am a middle-of-the-road sort of Christian, but have experience with both ends of the spectrum (very liberal UU Christians and very conservative Baptists) and have found that understanding people's motivations and beliefs helps me get along better with all of them and respect them, even when we're "different". Understanding them helps ME cope.
Ykwim? It would be easy to turn differences into fodder for debate and fuel for arguments. But unless there's something to be gained from it, I wouldn't waste the energy.
Bottom line: Your sister is coming for a visit. Enjoy the visit. If you are comfortable going to a church of her choosing, by all means go. You don't have to convert just to go listen.
But if going to her church of choice will make you uncomfortable or violate your own personal beliefs or standards, then do not go. And when church is over, try to get past the religion thing and enjoy your sister for who she is. Perhaps you can make a polite truce to keep religion out of the conversation if it causes strain?