Brother has a dual citizenship

I am about to begin the top secret/SCI with full-scope POLY clearance process. I have a question about the following:

My brother is a dual citizen of the US and Lithuania. But first, some very quick background about him…he was born and raised in the US. My parents were born in Lithuania and came over here when they were like 10 years old…thus our ethnic background. Several years ago my brother wanted to travel to Lithuania and learn abut the culture and research where my parents were born. Rather than get a VISA, he decided to get a citizenship. This would not only allow him to travel to and stay in Lithuania but also in the entire European Union freely. His only foreign ‘contacts’ were with the Lithuanian Embassy in Washington DC, the Lithuanian archives, and the Lithuanian emigration department.

Can this is ANY WAY adversely affect my eligibility for the top secret/SCI with full scope poly clearance ?

Yes, you will most assuredly be questioned about the concern because of the SCI access part. The are always issues when anyone has immediate family members who hold citizenship from other countries, especially wen they sought it out to gain a benefit. It can be mitigated, depending on the agency and your position, and will require you to provide additional information to help mitigate the concern.

How can something like this be mitigated? I feel like my only option is to ask my brother to drop his citizenship. I would hate to ask him that though since he had planned on traveling Europe and he said traveling with a citizenship would be much easier than traveling on a visa.