brother/sister?? dont think so, yet. konan, nagato, yahiko all lost their parents in war. but it doesnt say anywhere in manga that konan, nagato share their parents.
also controlling bijuu and sealing bijuu are two different things. 1st could control any bijuu but 4th sealed it. remember chyio was the one who sealed shukaku in gaara. but in chiyo's case she extracted bijuu from another jinchuriki i guess and sealed within gaara.
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if nagato was somehow related to 1st (i highly highly doubt it), he wud have controlled bijuu, not sealed it. although i think controlling bijuu doesnt make anyone relative to 1st. for example madara controlled kyubi, that doesnt make him relative to 1st.