Mother’s with Infants

So I’m having a difficult time bein a new mommy to my 6 months old son , while living with in-laws and their 7 year old . Kids love babies I know but can be rough with them as well. The 7 year old shook my sons head wild I really wanted to lose it but I didn’t and they made me feel like I couldn’t say not to do that to my baby .. I don’t want to cause friction or make it seem that I don’t want the kid to be around my baby . What’s some good advice?