Well, the good news is that DreamHost doesn't put any of those 'pesky' ads on your sites The only ads you will see on your site are the ones you place there.
However, DreamHost doesn't offer any kind of easy to use site building application. You are expected to have a certain level of knowledge when it comes to creating your webpages and uploading them.
Having said that, the whole procedure isn't too hard and there are many easy to use HTML editors out there (some are free). It might pay to have a look around and see if you can find one that you like, you will then be in a better position to decide if you you can get your sisters site up quickly enough.
As for uploading using FTP, this is also pretty straight forward. you have a number of options here, you can use a proper FTP client, the Web-FTP provided by DreamHost or even just use IE (if you are using Windows) and drag and drop the relevant files directly to your hosting space.
Overall, DreamHost is probably not the best choice for the absolute beginner, especially one that needs a site up and working fairly quickly, but they do provide enough resources (space and bandwidth) that you can grow your site(s) without worrying too much about running into quotas etc.
If you do decide to go with DreamHost, you will find this forum a great resource if you run into problems or have trouble working something out. There are many helpful individuals here that are sure to lend a helping hand.
Whatever you decide, I wish you good luck and hope things go well for you.
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