I haven't really had a long time to test it, but I had an alliance with the Polish. I got a princess and married her off to the faction heir of poland, when I mouse over the little faction description where it shows reliability and relations it now shows that we have an alliance and 1 marriage between us. I'm hoping they honor it, I have quarrel with the Polish nor was I planning on it. Perhaps a marriage can strengthen a current allaince, but a marriage between two factions who were not allied previously is not as strong?

Kind of like the point system for guilds for example. having an alliance is 10 points towards not stabbing you in the back. Having an alliance then adding a marriage strengthens that bond? I don't know for sure, I'll have to see how plucky the polish are feeling in another 45 turns or so.