When you say adult you mean they are older than 18? If that's the case, you can't force them into therapy. If it's hard to convince them that they will benefit from it, then, unfortunately, there is nothing you can do about it. All I can say is that you might have a chance to convince them if you talk to them very nicely and let them know that you believe it'd be good experience for them. If they feel that you are pushing them, they won't do it. Another thing to try is to ask if they'd be willing to attend a family therapy session with you together so you could find out why they object to therapy. But if they say no to that and no to therapy all together, then you'll have to accept that.