GF and sis avoided telling me?

Lats fri i ws so tired after wk and decided nt to go to the bar that my gf and i usually go to on fri w our friends....bt my sis went like usual( i told her specically to tell me if ths girl Mary gs). The next day my sis tells me abt the night before and didnt mention mary bt descirbes everybdy wlse. then this sunday, i speak to a friend that ws sthere and said mary ws there so ia sk my gf y she didnt mention it and she said it slippd her mind thn i ask my sis and she said she forgt and that i wld hv gtn mad neway if i knew mary hd shwd up. i feel liek they r keeping thngs frm me...my sis and grl. bc the day b4 my grl slippdand said my sis hd came ovr the last wk whn my sis said ahe didn't. i knw ppl r gna say they r cheating togetehr bt truast me thats NT possible. anyways, is it wrng for me to be mad that they faild to tell me? i just feel like my gf must keep other thngs frm me so that i dnt get mad.....