Her brother shouldn’t be getting involved in the relationship you’re trying to build with his sister. He’s interfering and quite clearly making you uncomfortable, so speak about it with her.

You have to be open with her and allow your girlfriend to know (in case she doesn’t already) – You also have to be honest with yourself. What caused these trust issues? Was it a past relationship? Was it something you did that you believe she may be capable of doing to you? Figure out the root and become aware of it, making sure you don’t reflect it on to her.

He’s clearly being immature about the situation. Leave him alone till he’s ready to come back to his senses. You’ve done nothing wrong so you shouldn’t act like you have. When he does eventually come around, you need to have a serious discussion. Asking for a break over something like this is petty. You should be able to have honest and raw conversations without being able to totally cut each other off.

You need to let him go.

You’re going to have to build a bridge and get over it my dear, you knew of is circumstances and it’s not going to change. Some people can cope with their partner having another family, others can’t. Perhaps you might be the ‘others’. Think about it and ask yourself why you feel this way. You mentioned the fact that him and the mother of his child do not get along, would you be unhappier if they did?