I have a relationship with my ex's sister and tell me is it abit weird?

[J.v.T.k.]

I'm 16 and I currently have a 5 month relationship with my ex's sister and my gf is 16 too, thing is I went out with my ex last fall but it didn't work out we lasted about a month or so, then on Feburary 10th 2009 that was when I met my ex's sister and we have been together ever since, I've even stayed with my girlfriend for about 3 months at her place, yet I saw my ex at her place too but we didn't talk to each other for abit but we're talking now she currently has a bf too and my gf's mom knows I dated her sister before I even met my gf, but everything with my gf is going good and her family and younger siblings like me staying there, so tell me is it alright doing this? give me your answers