Well, the Scottish custom was for non-Scots (or those not affiliated with a clan) to be adopted into their wife's clan. That being the case, and if Erskine is your closest Scottish ancestral connection, then it would be reasonable to consider yourself a member of clan Erskine.

Since a fair amount of time has passed since your ancestor married into clan Erskine, I would suggest that you establish an "official" tie to the clan. The easiest way to to do this is to write to the Chief of Clan Erskine, explain your situation, and asked to be recognized as a member of his clan. Once the chief welcomes you into the clan, you are good to go.

Now there is really no reason to do this, other than simple courtesy. There is nothing to stop you (other than good manners) from wearing any clan badge or clan tartan, including Erskine. But if, as it seems from your original post, you are approaching the subject with respect for Highland tradition, then asking permission of the chief to wear his badge and his tartan would seem to be the right and proper thing to do.