Do I have the right to be annoyed?

My SO's mother is very welcoming and tries to help people in any way she can, which is great don't get me wrong. However, my SO's older brother has a friend who got kicked out of her house. So their mother so kindly welcomed this girl into their home, which I was already a bit annoyed about because she'd only met the girl once and the only thing she knew was that she needed somewhere to go, but it wasn't my place to step in with my input. So now this girl has been moved in with them for about a month. She leaves her clothes and other belongings ALL over their basement, eats all their food that their mom struggles to get as it is, brings in people that me, my SO, and his brother have had problems with before, and lastly she hasn't showered since the first week she's been there. It just bothers me because I've dreamt of living with them for the longest, and they've always told me how welcome I am there, but no matter how welcome they make me or anyone feel, that type of behavior is not acceptable from ANYONE! Its irritating that my SO's mother seems to have her head so far up this girls ass, but she flips out around everyone else because of what this girl is bringing to the house. I honestly feel like this girl is so disrespectful and it angers me that their family has been so welcoming and this is what she does.