Minecraft

Hi,Idk who to go to but whenever I try and launch my Minecraft with mods its crashes and crashes with not mods,i reset my laptop fully many times and still has the same issue but whenever I do it on my sister laptop & log in and we have same laptop it works! it also crashes when I join games, also I have another bug whenever my sister log into her mc and her account is rly old she only use to log into her mc account with her username (IGN) & password and now she can only log in with her username & password and can never change her Minecraft skin! any help?