Stepmom between rock and hard place

I am married to a man who has two children from a previous marriage and we live in the state of Virginia. He has custody of oldest son (who lives with us) and ex-wife has custody of youngest son (who lives with her). We also have a son together (4yrs) who lives with us as well. The ex-wife constantly encourages the oldest son to misbehave, claim being abused, etc., so that he can come back to live with her. She also has a consistent pattern to make other allegations (about other family members, i.e. molestation and abuse) towards the oldest child - but when the child is questioned, he denies ever having talked with this to the mom. Social Services has now become involved over the molestation allegation she rendered. I want to know what position this has placed 'my child' in, our 4-year-old. If they do 'remove' the oldest child from our home, will they also remove the 4-year-old? There are no complaints or further allegations which concern the 4-year-old at all, nor are there any complaints filed against me personally.

All of this is simply just a mess over a string of untruths and an ex-wife that simply just won't let well enough alone and is not happy unless someone else is miserable. But, in the best interests of my child, I need to know how this could possibly affect him and what steps I need to take to ensure that my child stays with me. Thanks in advance!

It will depend on family services findings and how agressive the social worker wants to be.
If they establish that molestation has taken place in the home, they are obligated by law to look at all children living in the home.