Needing some family advice...

I need some advice out there...I am trying desperately to cool off before broaching this subject with my sister.  First off, my sister is married with very young children and I am not with quite a bit older children.  Anymore when I am around my sister she takes it upon herself to critize my parenting.  For instance, my youngest plays sports, but this year it is a semi-traveling team.  I have a used car, compared to her new one.  I cannot afford a new one right now or a couple of repairs that need to be done.  A couple the sports meets will be an hour away for me to drive.  I mentioned that I just wasn't sure we would be able to catch every away game this season, missing maybe one or two.  She informed me that my youngest was not asking for anything major by expecting to go to all of her games, but it was my "lifestyle" that's the problem.  She also mentioned after driving an hour each way last weekend to her child's birthday party that although she had invited my child to come for part of the summer with her she wanted to know how I could tell my child that she might not be able to go as much as she would like this summer as the trip is expensive for me (about $15 in gas each time I go to my sister's) and the car really shouldn't be taken that far right now very often.  The conversation is always that I am cheating my children out of things they are entitled to as children.  She even told my youngest we would have an "unbirthday" for her this summer as her birthday falls in the winter and usually it is too nasty to have a party outside and such.  I did say sure at the time, but as time passes paying for another party for my child right now is really going to be financially pressing.  I feel as if she is insulting me and should not extend invitations to spend the summer or have unbirthdays and expect me to just come up with the money each time to do these things.  Sometimes I have extra money and sometimes I don't.  Money in this business does flutuate and things do come up.  Acccording to her, I should find another job so that I can do the things my child "deserves".   With this job, sometimes I can be off for special functions and sometimes I can't.  I feel that sure if I had a husband who works and does side jobs I would have more money too, but that is just not what my life is like.  She feels that if I chose to have children I should have everything provided to them that "every other child gets to do".  Am I off base here?  We have all been trying to get together to do more family stuff and I am trying extra hard to arrange my schedules and bills to accomodate doing what her family does on her husbands income and her part-time income.  How do I approach this without starting a major arguement?  My mother is no better and I have mentioned to my sister that I have considered not seeing my mother as much when she is negative and critical.  My sister is very vocal about not "cutting off family", but I am so tired of swallowing my anger and feeling degraded and insulted after every get together.  I know my child likes to spend time with grandma and auntie and her kids, but this is getting to be more  than I can take.  I was up the other night until the wee hours of the a.m. because I was so angry.  PS. She also has a "college degree" that she does not use and took classes in child development so of course she knows all.  My kid gets the sniffles and it is "well if it were my child I would be taking her to the doctor".  I'm not the type to panic over every viral thing my kids get.  Drs. cost money and most of the time unless there is a raging fever involved or a somewhat lengthy illness, I ride it out before I cough up $110 for a Dr. visit.  I am ranting now, but what do I do here????  Any advice is so appreciated before I just get angry and handle this all wrong.