My sister is bad at handling things and keeps losing out.
Literal
Younger-sister [topic-は] tact [subject-が] bad-and loss only am-doing.
要領が悪い ('bad at grasping essentials, tactless') is a set expression — 要領 ('the gist, the knack') + が悪い ('is bad'). 損ばかりしている ('keeps losing out') uses ばかり to emphasize that losses are the only result.