Pentacles also relate to the element Earth. In the case of a relationship spread you can look at things from that perspective. Some of the Earthy qualities that you can use in a relationship reading are things like (on the positive side) strength, stability, solid, supportive, and (on the negative side) rocky, shaky , unstable.... and the list of both positive and negatives can go on and on.
For example:
The 2 of Pentacles often is balance of some sort. From a relationship point of view, you can see it as an equal give and take from both people in the relationship.
The 3 of Pentacles (someone who works hard to achieve their dreams) can show the dreams of a relationship (possibly marriage) coming to fruition; especially when paired up with the "right" cards from other suits.
The 5 of Pentacles, which often depicts hard times, can show a lack of support from someone in the relationship.
As usual though, all the meanings will change a bit depending upon the other cards around them and the position in the spread. The examples I gave above are just looking at 1 card at a time and not looking at them in a spread.