Found: YA/Kids, Boy, Dog and Sister. Wizard/Warlock

Im looking for a book thats at least 12 years old, it has a yellow and black front cover with the sillhoutte of a suit of armor and a castle. Its a series but i never read the first one, but i gathered from the second one that at some point the boys mother (while pregnant) swam in dragons blood and now the baby sister Matilda aka Tilly is a super powerful. special magical being. The family has to go to wizard/warlock trials and all hell sorta breaks loose. The boy, his sister and dog Tink are taken somewhere "safe" by the warlock or wizard prime minister (His name is Justin) they end up at a magic castle that changes layout every night. they explore and find a suit of armor that came to life, they climbed an ivy wall of the castle to escape it. they discovered a sort of prison where like goblins or trolls are (cant remember what species they are) and the boy meets Iggy whos really good at lock picking and uses a tooth brush to break locks. they end up being captured by Justin who turns out to be the bad guy and is a warlock with black tipped ears and everyone gets put in magic absorbing collars. normal happy ending sort of thing. Some one please help, this has been driving me crazy for years! TIA

The Castle of Creepiness! / Measle and the Mallockee by Ian Ogilvy.

omg thank you so much! ive been trying to find this for so long