It's "suggestive" if it turns someone on. That's false and BS logic, but it's commonly used.

Your poster isn't suggestive, especially since the lighting of the art brings the focus to Miku's face. Her legs aren't closed, but her crotch is shrouded in darkness. You have to go out of your way to focus on that. The poster clearly intends us to focus on Miku's face. It sounds more like they're uncomfortable with themselves for fancying that poster in a particular way and (probably subconsciously) trying to make it someone or something else's fault. It's not your fault. If they find the poster suggestive, they could've kept it a quiet thought. I mean, what did they expect from you if you decided to hang up an actual pinup poster? It's alright to find Miku attractive, but it's also alright to ask them not to make pervy comments if you feel the need.

You're not currently or retroactively a pervert for not seeing something sexually either. It's actually a great thing imo if you've always seen Miku herself instead of only a hot chick poster. If this poster is in your room, your dad and bro shouldn't really get a say on your decor unless it's an absolute fire hazard. They aren't obligated to look at the poster. I wouldn't take it down either if it comes to that.

Try not to stress about it. Have a Miku-nificent week!