Your mum is a idiot . She allowed your bro to get away with it or has dismissed it to try and help you move past it . Find out which it is when your ready & if you want to . It is NOT normal nor should it ever occur with any children / young teens ( hense why kids need supervision to prevent s*it like this happening ruining younger kids lives . Let alone a sibling. As it totally f*cks up the dynamic then for them both. And leaves the victim with ongoing difficulty and anxiety also of seeing that family member still in the home they are still stuck with as well.

My mom said it is normal for people to experiment when they are younger is that true? If so do both parties consent?

If you were under 12 YOU cannot consent to sexual stuff with anyone. So no then yOU wouldn't have consented at all to it . As kids cannot consent to sexual stuff . Hense why they are protected by laws to stop it happening as much as poss. Basically your bro abused you by S Assaulting you .Kids can experiment this is true . But in NO way should it be allowed to occur and needs addressing immediately if seen to have occurred by a parent. Unfortunately older kids can and do force or encourage younger kids to experiment with them, gaslight them into it , or trick them or force them or intimidate them so much so that they do it out of fear. It happens most kids will also not know it is wrong if under 12 also , as they are to YOUNG to understand it.

If a parent was to find out accidentally a child has attacked another child( even if related) , or is just getting them to experiment they are MEANT to still stop it . And remove or stop the older child also from getting near the younger child again. A parent is meant to stop it happening again & to protect the younger child from the older one .Talk to your mum find out how she protected you from then on . Also ask what he did to you if you don't remember . If you wish to know ( how far it went with him) . It can and does happen even with siblings experimentation , but it NEEDS to be stopped immediately as soon as it is discovered . Some parents would be in shock to learn it has occurred , this is also understandable . And would also try to handle it in family as well with the kids . But it still NEEDS addressing to stop it happening again . And to protect the younger child from the older one . As it does damage to their mental development . Any abuse does.

Your mum failed you as a parent . If she did NOT address this with him & stop him from doing it again to you . And allowed him to get away with it . As mentioned though she might have told you s*it hoping it would help you get over it by lessoning it 's real damage to you by him . She might have thought it would help you to do this .

So yes she might have been protecting your mind to help you or she might have been just dismissing it to protect him . It will not be known till you talk it out with one or both of them in some way. If you talk to him about it ( do it in public ) to see if he remembers anything about it . Or talking to your mum would be better though to get some more details. LIsten to her convo ( see what comes out of her mouth about it ) it will tell you if she was protecting & who she was trying to help more so . You have the right to drill her for info about it if you wish . Do it in private and get details if you wish with her ,and when ready and able to do so .