Who is in the wrong here?

Well what else did you expect him to do?

My boyfriend is away on a 'long holiday' and will not be back for the foreseeable future. However his sister is really ill and everyone has been hiding it from him. I told him on the condition that he keeps it between us. I didn't think it was fair for him to not know about how bad things are, and he promised not to mention it to anyone. However he literally IMMEDIATELY called his mother and told him everything I'd said. Is is understandable for me to be pissed off?

Well what else did you expect him to do?

Did his sister also want it hidden from him?

she doesn't care - it's only his mother who didn't want him to know

He'd promised not to mention it to his family because they've been hiding it from him - he promised to wait until they mentioned it to him

Screw what the mother wants, his sister didn't care so you shouldn't feel bad about telling him. You are not in the wrong

This is what I thought - his sister doesn't mind but his mother has been going to great lengths to hide it. I feel upset that he broke my trust, because the condition was that his mother doesn't know he knows

Although hes in the wrong for breaking your agreement, his reaction is pretty understandable and id probably do the same.
I doubt he really cares about whats fair and whats not fair atm, since all that matters to him right now is not being told his own sister is really ill.
Although you might be upset (quite rightly so, breaking promises isnt very nice no matter how big/small the thing is), I'm assuming he just hopes you understand why he did what he did and let it go.

Update: I pulled his mother over and explained that I'd told him and she said I did the right thing, because somebody needed to tell him but she was too scared to. So she's grateful to me for biting the bullet and doing it