Category.DataBind();
... other c# code ...

In your .aspx file you will have to change DataSource to your child repeater (SubCategory) and syntax in which you get data.
The code below does so:

... other HTML code ...
<%@ Import Namespace="System.Data" %>

<asp:Repeater Runat="server" ID="Category">
<ItemTemplate>
    <a href="category.aspx?category=<%# DataBinder.Eval(Container.DataItem, " rel="nofollow"Category") %>"><%# DataBinder.Eval(Container.DataItem, "Category") %></a><br>

    <asp:Repeater Runat="server" ID="SubCategory" DataSource='<%# ((DataRowView)Container.DataItem).Row.GetChildRows("CategoryRelation")%>>
    <ItemTemplate>
        <a href="category.aspx?category=<%# DataBinder.Eval(Container.DataItem, " rel="nofollow"["SubCategory"]") %>"><%# DataBinder.Eval(Container.DataItem, "["SubCategory"]") %></a><br>
    </ItemTemplate>
    </asp:Repeater>
</ItemTemplate>

</asp:Repeater>
... other HTML code ...

If you do both changes, code should work fine.

Another interesting thing to note is how you can access a dataitem from parent (from within your child).
For example, we wish link to include Category also, even if it is a SubCategory.
Adjust your .aspx file to following:

... other HTML code ...
<%@ Import Namespace="System.Data" %>

<asp:Repeater Runat="server" ID="Category">
<ItemTemplate>
    <a href="category.aspx?category=<%# DataBinder.Eval(Container.DataItem, " rel="nofollow"Category") %>"><%# DataBinder.Eval(Container.DataItem, "Category") %></a><br>
    <asp:Repeater Runat="server" ID="SubCategory" DataSource='<%# ((DataRowView)Container.DataItem).Row.GetChildRows("CategoryRelation")%>>

    <ItemTemplate>
        <a href="category.aspx?category=<%# DataBinder.Eval(Container.DataItem, " rel="nofollow"["SubCategory"]") %>"><%# ((DataRow)Container.DataItem).GetParentRow("CategoryRelation")["Category"] %> - <%# DataBinder.Eval(Container.DataItem, "["SubCategory"]") %></a><br>
    </ItemTemplate>
    </asp:Repeater>
</ItemTemplate>

</asp:Repeater>
... other HTML code ...

This code is both easy to maintain, and performant.
The same principles count for other items bound to database data, like DataGrid for example.

Large for profit websites can afford to hire web-orientated staff to control, advertise and spend time on boosting rankings of their particular website. Single owner for profit websites can afford to pay marketing specialists and to buy keywords at exorbitant rates. The lone not-for-profit website owner can either spend all day and all week controlling and submitting his site to thousands of ever changing search engines and directories and suffer from a serious loss of updated content on his/her website or place emphasis on building up content and never have a visitor to his portal.

Many other factors go towards reducing effectiveness of individual website than just search result rankings. Many single website owners operate outdated and very slow computers, use old or outdated software and only perform on odd occasions when not playing with their children or busy at work. Many other free info sites have found that subscribing to some lists to boost their rankings has in-fact reduced them to near invisibility. Google and now other search engines condemn sites for using link pages that they themselves do not agree with, so by simply subscribing or joining one of these sites Google may drop a future crawl of website involved. It is also impossible for lone not-for-profit website to keep up with ever changing trends and policies. Where payment is made for a lifetimes inclusion in a search engine, next year may see demise of this particular engine or its partnering up with another – thus lifetimes inclusion becomes null and void and to prevent being dropped from listings another fee is required – read small print!

Other means to increase visibility is often initiated by offering advertising space to companies like Google, Barnes and Noble or other directories or affiliates. This can result in a slight income for websites (The Seamania website made 40US dollars over last three months) but never enough to afford placement on search engine results or to purchase keywords. It is also against grain for many free-info website owners to have to place advertising on their websites as not only is it taking up valuable space it detracts and reduces free effect content within. Furthermore should a website choose one companies advertising it may boost their rankings within one search engine but equally so reduce it in another’s e.g. allowing Google advertising space on an index page may increase page rank in Google but seriously reduce it in Yahoos search results and possible exclusion from their Yahoo Directory.

From point of view of an Internet User in search of free and not-for-profit biased information he/she does not want to see endless sites where a visa card is required to proceed further. It would be very nice to see advancement of such directories like Zeal.com who divide their listings into those for profit and those who generally provide valuable and non-profit orientated formation. Naturally question arises as to how such a search engine would manage to cover costs of these listings but generally with amount of people available who regard Internet as a toy and a hobby projects such as Editor of a category volunteers should not be hard to recruit. The other way would be to have search engines run and operated by governments like public libraries are or built and operated by universities as part of study programs – something practical for students to involve themselves with.

Directories abound whose content is managed by volunteers, Open Directory Project being most famous. But sites such as Seamania have found to their detriment that trying to get noticed in amongst debris found in these directories is not easy. Seamania was originally listed as a Personal website in boating category but over time emphasis and content of website has evolved to become a general travel website. It has though proved impossible to change location of site in Dmoz directory to a travel listing rather than a boating listing.

If at all possible and to prevent users who are sick of being asked for their credit card information or being given 30 different porn sites upon entering Travel Tales of Sea into a search engine, it would be nice to see a shift in emphasis in way that search engines operate their listings.

Certainly idea of switching on a computer and being faced with two choices, one for sites that are-for-profit and one that points towards not-for-profit sites would be a dream come true. To enter in a search request and to not find amazon.com or ebay.com in first few results would put cheer to any searchers hopes of finding what he wants. And maybe in this way a true exchange of information may be facilitated and mass exodus of searchers back to public library for information may be halted.