salams sis.. if your dad is willing to go ahead even without ur mum's support... and as long as the "sect" the brother u wanna marry is not a misguided one (put simply as long as the bro u hav in mind is following QURAN and SUNNAH) then GOOOOOO for it and inshalah ur mum will get over it.. and if she doesn't, it's still your right and just be really ncie and all to her even tho she might threat to cut ties and what not....

but the keyword is 'sect'... its nto a nice word to use, but instaed of looking at wat sect he is.. make sure he is following what Allah and the messenger have to say.. nevermind name calling coz it stinks.