I'm on a roller coaster ride with birth mom. Advice?

My little girl (5 years old) has been with us for 3 years now. It was agreed that it would be open on the condition that the birth mom would have to request visits, call, etc. I have no problem with that. However, birth mom goes through ups and downs in her attention toward the child. Months will go by when she calls and requests visits often. Then several months (up to five) go by with no calls no attention no requests. This roller coaster ride tears up the child. On one hand she get progressively angrier each time but on the other hand she makes up excuses for ther other Mom. Last year this happened and I spoke to the birth mom about it and we agreed that she would make more effort. Recently, for the last 4 months there has been only 2 phone calls and no visit requests. When I brought it up to the other Mom, she wrote a scathing manipulative letter blaming me. Then she covers it up at the end with a nicey nicey sentance saying she wishes we could talk about this. Oh, she also has 2 other children living with her and child asks why they get other Mom's attention and not her. This is what I need advice for: 1. Which side of my daughter's feelings do I support? Do I let her make rediculous excuses for her other Mom? Or do I just tell her it's ok to be angry and not let her make excuses? I am sick of making excuses on birth Mom's behalf and won't do it any more. 2. Do I answer the letter from birth Mom? I haven't yet because I'm so angry, but she is just trying to alieve her own guilt and blame somebody else. I am not responsible for her. If she does not have a relationship with her daughter because she does not make an effort, it is not by problerm. Do I tell her that? 3. How do I explain the 2 siblings and the attention they get? Wow, this is long, but getting alot off my chest is rather freeing. Thanks to all.