Anyone know how to convince a stubborn parent to get a small pet?

My sister and I have been asking to buy a small rodent (with our own money!), but he says no. We have asked for mice, hamsters, and gerbils! It is not the indecision that makes him say no. He says we are irresponsible and he just doesn't want one in the house. But his parents let him get any pets! My mom agrees with us but she cant convince him. The only he says we can prove our responsibility is to like keep our room clean and not fight for a whole month, which we can od for a short amount of time andn a short amount of time only/ Please help!!!!