advice ??

hey ladies , my bf brothers gf had a baby a few months after I did. my son's now 14 months and only drinks out of a sippy cup. well his sister in laws son is 10 months on milk and sippy cups also . well my mother in law ALWAYS buys this baby something. she went to the store the other day and she told me this is HIS sippy cup being he don't have one here i go okay thank u. well the sister in law sends me a snap of her son chewing all over the cup my mother in law just bought my son (which he used 2 days ago). But agian now her son has his own cup for her house also she let her baby chew on the cup my son just used how would u feel ? it's like how does she know that I didn't forget that cup on accident and now she's using it. to me that's gross.. advice?