In a mass spectrum, are there peaks corresponding to a combination of 13C and bromine isotopes?
It does appear. The graphs you're looking at online probably just dumbed it down.

Here's some proof if you like, a compound synthesised by yours truly, with molecular formula $\ce{C7H7BrN2O2}$. I clipped the relevant part of the mass spectrum I got. This was done with ammonia gas CI, so instead of the $\ce{M^{+.}}$ radical cation we expect to see the $\ce{[M + H]+}$ quasi-molecular ion. We expect the following monoisotopic masses (I used this website to calculate these):

$$\begin{array}{ccc} \hline
\text{Molecular ion} &amp; m/z &amp; \text{Predicted relative abundance} \\ \hline
\ce{[C7H7^{79}BrN2O2 + H]+} &amp; 230.9769 &amp; 100.00 \\ 
\ce{[C6^{13}CH7^{79}BrN2O2 + H]+} &amp; 231.9798 &amp; 8.48 \\
\ce{[C7H7^{81}BrN2O2 + H]+} &amp; 232.9749 &amp; 98.01 \\ 
\ce{[C6^{13}CH7^{81}BrN2O2 + H]+} &amp; 233.9777 &amp; 8.29 \\ \hline
\end{array}$$