Do ninja clans have to keep relations only within the clan to ensure the inheritance of abilities? Are there any references to arranged marriages or forbidden relationships in the history of clans being used to avoid the clan abilities from being lost?
As mentioned in the series, the clans usually do not want their "powers" to fall into someone else's hand. As these could be used for research against this power or getting even greater power for the clan that obtains it. Therefore they try to keep relations within clans. Then again  relationships with different clans do exist. Naruto's parents would be a good example for this  as Minato Namikaze married himself into the Uzumaki clan with Kushina Uzumaki


  Membership in a clan is usually determined by blood relations and
  genetics, which is vital in the usage of Kekkei Genkai and secret
  techniques. These attributes and abilities made many clans feared and
  renowned throughout many villages. While clans may be composed of many
  individuals and extended families, many clans mentioned and seen in
  the series were somewhat limited to a nuclear family. Wikia


Also this part of the wiki states that family's (clans) can be composed of many families.

As for forbidden relationships, yes they existed (not always in a romantic way ) Look at the first Hokage and Madara Uchiha. They were best of friends yet their family's did not accept their friendship. So if I were to guess I would say that there is a high probability that there are also forbidden relationships 

I couldn't find any specific information about forced marriage´s so far. But if I do find some information on this I'll add it here later.