MIL problems

My SO's cousin had her baby like 3 weeks ago. She asked to borrow our sons extra car seat to bring the baby from the hospital and we said yes. She continued to use it after the baby was out of hospital and of course that was okay. They originally live in Tijuana and were planning to stay here for a few months, but out of nowhere they decided to leave yesterday. My MIL text my SO to see if they could borrow it again and that they would "bring it back in a couple of months." He was at work so he didn't get a chance to respond, she didn't bother to call or text me at all and just decided to let them borrow it. It upset me that she didn't bother to ask me, like if my opinion didn't matter. She's upset and doesn't want to talk to us now and she called us greedy basically, "fijados" in Spanish. Am I in the wrong? Am I supposed to be okay with her taking my sons stuff as they took his bouncer as well without asking for permission? I truly don't get it, but if I'm wrong I would like to apologize but I don't think I am.