Is it normal getting used to bjs by licking food off my brother?

So I've been with my boyfriend for almost a year now and he really wants me to give him oral sex. trouble is, i've never been able to bring myself to do it, i get scared and back out. i want to make my bf happy but i just couldnt do it.

so i asked my sister for help and she said that one way to get used to it would be to lick food off of his cock, that way i can focus on the eating rather than the blowjob. so she called in my brother and told me to practice on him before trying it on my boyfriend. i was so shocked, and so was my brother too.

we started by having me lick peanut butter off of his arms and hands, legs and feet, to get me used to licking someone's body. but then my sister put it on his crotch and told me to lick it, and i started to. then i heard my brother moan and it got super weird and awkward. this is my brother we're talking about and i'm supposed to give him a blowjob? i mean, i know my sis is just trying to help and everything, but this seems really strange to me. (by the way, i'm 18, bro is 22, sis is 25)

its been almost a week now and i can only get in a few licks on my brother's shaft before losing my nerve and backing out. i also foolishly asked my sister to show me how a real blowjob is done and she got embarrassed, cuz she had to show me using our brother. it was just really strange and awkward watching her suck him off, seeing his satisfied face yet knowing he didnt want us to be doing strange things to him. (he says he doesnt think what we're doing is right but he sticks with it because he wants to help me)

does anybody have any suggestions on how i can get used to giving blowjobs? please, its for me and my boyfriend's sake, i beg of you. also, is the way me, my sis and brother practice to help me get used to it normal? should we keep going?