What have you got to lose by going to Relate? Yes, it might seem pointless now, but, if it didn't, would you really need to go?? What I mean is, counselling might just manage to resolve a problem which seems insurmountable now. If you are communicating by text messages, perhaps the real communication in your relationship is failing. Counselling will give each of you the opportunity to say what is important to you and, if you still feel there is no future in your relationship, so be it. However, you may find that you manage to communicate so effectively this way that you can talk your way out of an impossible situation.
Don't blame your partner for his sisters' attitudes. My SIL hasn't spoken to me properly for nearly 5 months now and I have no idea why. I certainly don't blame dh for it or expect him to sort it out. If her attitude was because of something dh had said or did, then it might be different.