MIL wants me to my husband in separate rooms

Me and my husband are planning on going to a festival up state. Many of his family members are going. My husband asked his mom if we could stay with her to save money on a hotel. She said yes but only if me and him sleep in separate rooms even though we are married. She's still upset about something that happened on Easter. My husband is the oldest. He has 4 siblings and when we went down there to stay for Easter we shared a room with his little brother who was 14 at the time but turned 15 recently. We stayed for two days and on the last night his brother was asleep. His brother takes medicine to sleep and it usually knocks him out so my husband started fingering me and we were fooling around like married people do. We didn't know that we woke up his brother and his brother told my MIL and when we left we got a test from her calling us disrespectful and that his little brother let us in his space and we did that. I don't think it was that big of a deal. We are married. She knows we clearly have sex but she was angry about it. She seemed more angry that my BIL heard it. But 1. He's 14 and knows what sex is and 2. We didn't know he was awake. Now of course if we knew we would NEVER have done that. But my MIL was pissed and told us we have no respect. It wasn't our room and we were guest in both her house and her son's room. We thought the whole situation died down but when my husband mentioned staying with her she said if we stay in different rooms. Even though we are married. I could maybe understand if we were dating but we're married. She said we broke trust so that's the consequences. We aren't kids. We are adults who have a home and job and the fact that she's trying to punish us like we are teenagers. My husband thinks we may have to get a hotel but we may not be able to afford one. So we either don't go or go by his mom's weird rules.