Advice on Family/BF problems

I'm worried. my boyfriend is coming to see me again in 2-3 weeks after a few months of not seeing each other and he's going to meet my family. my body and my boyfriend's body are fit, because we're just that fit couple. but the real problem is my sister...she's 2 years younger than me. she has an ass, bigger than mines. she's not so fit..because her dads side of the family is a bit chubby with a big booty..but again, she has an ass. all that im afraid of is him seeing that. just his thoughts. no freaking rude comments because I'm sick of those rude users. just some advice, to talk to him, avoid that, etc.?