Once your sibling files the immigrant petition for you, you cannot go back to Nepal and come on non-immigrant visa like F1. If it is H1B it will be fine since H1B is dual intent visa. The intent of applying while in US is that you should stay in US while maintaining legal status till you get the green card or at least Advance parole before you leave US. 

If you leave US and went for new F1 visa or come back with your valid existing visa, then even if POE officials didn't find about ur petition (and you didn't reveal at POE or during F1 interview abt ur immigrant petition) and you are able to enter US in F1, you will be considered as committing visa fraud (coming in non-immigrant visa by not revealing to officials that you have been already petitioned for immigrant visa) and you will have a huge problem during getting the green card. 

And if you reveal that you have been petitioned for immigrant visa thru ur sibling, then if you go for interview for F1 visa, they will not give F1 visa and will tell you to wait that 12 years to get the card or even at POE they will possibly send you back since they cannot let you in with non-immigrant visa. 

Hence the best is to be in United States, maintain F1 status or be in H1 status for the entire periond.