Australian daughter in law rarely acknowledges anything that I do for her.

My DIL (35) and Canadian son (34) visit my husband and me once a year. Although I really try hard to do nice things like pay for meals, play and babysit their 3 and 4 yr old kids (my grandkids), buy books and toys for the kids, fix meals, clean house while they are visiting, she rarely says thank you (she never offers to help despite staying for 20 days at a time). I stay silent because I see them infrequently, but it makes me feel uncomfortable. She would definitely not want to hear me say anything to her, and I have mentioned it to my son years before. I know it is my fault for feeling a bit hurt, but I am wondering if this is how Australians are with their family as I have noticed her thanking restaurant staff when they attend to her. Please enlighten me.