What's the age of your gf and his brother. You said your gf is a teenager means at max she should be 16 or 17. Then her brother must be 14 or 15. And your son is 12 years old..you are doing sex with a kid who is about your sons age ..exactly 3 or 4 year older than your son.
You should stop having sex with kids. Since they are kids or teen aged, they can tell your secret to anyone. Your secret is not safe with them. I know you enjoy when both of your nipples were sucked at the same time, it must be marvellous, mind blowing, amazing, exciting. You must be thrilled with a new vibration in your body.
I know it will be difficult for you to leave them but if you dont want to leave them and continue enjoying your threesome find a way to keep them quiet. If I get a chance to fuck 2 women at same time then I will also do it at any cost, and if anyone sucks ,chews and plays with nipple then nipples do enlarge..but it's a good sign. We men love big boobs ad large nipples with average pink or dark aerola
What's your boobs size and colour of your aerola? Since you enjoy when both of your boobs were being sucked at the same time, means you are bi. Bi means you love sex with men as well as women. Its the best gift that nature can provide and its gifted to special persons only. I guess you are beautiful, sexy,and most loved person made by god who is gifted to enjoy her full sexy, busty, lusty body with both man as well as women.
I wish you all the best and enjoy your body to the fullest and keep updating your feeling here so that we can utilise this platform at fullest and be success as you in sex.